A Model-building Study of Mitosis, I. Introduction

1963 213

A Model-building Study of Mitosis, I. Introduction

Yoshinari Kuwada1

Received March 1, 1963

Introduction

In the present study, mitosis and meiosis as a double mitosis are the main subjects of investigation. We understand mitosis as nuclear division. We know many details about it, but not much about the relation of mitosis to other types of division, such as chromosomal division and protoplasmic division. This relation is significant when examining what part of the mechanism these types of division have in common and what is essential for division. Again, little is known about the relation of mitosis to the division mechanism in bacteria which, since bipolar separation occurs, appears to be similar to mitosis. Such a study would be interesting to see whether mitosis and bacterial division are partially or entirely identical. Thus, we really know little about the true form of mitosis or about the fundamental principle of division underlying the mechanism in all types of division.

A study which might answer these questions is an analytical and comparative one, carried out with genealogical considerations of different types of division. From this, the true form of mitosis might be understood. However, as it is very difficult to obtain information on the true form of mitosis, we intend to propose an artificial model of mitosis, meiosis and some related

phenomena.A question which should be of primary importance in this study of mitosis

is that of the morphological nature of the nucleus-the vesicular shaped nucleus-the division of which we understand as constituting mitosis . This question arises because in mitosis the nuclear polar distribution is represented by a chromosomal polar distribution. It appears that there is a certain inconsistency in our understanding of mitosis when considered as a nuclear division, since divisions of structures of different orders-the nuclear division (chromosome distribution process) and the protoplasmic division (nuclear distribution process)-cannot be simultaneously represented by a single polar (anaphasic) separation;

each structural unit must have its own process of division or polar separation. We think that if we recognize the presence of the nucleus in the protoplasm, we must also recognize the presence of cytoplasm as its antithesis, and, therefore, that the division of the protoplasm must be a process by which daughter forms of the nucleus and cytoplasm become distributed regularly in two daughter forms of the protoplasm. This must mean that the nucleus, the cytoplasm and the protoplasm undergo distinct divisional processes. It is not possible to

1 c/o Botanical Institute, Department of Science, Kyoto University, Kyoto, Japan.

214 Y. Kuwada Cytologia 28

assume that nuclear division includes protoplasmic division, or that protoplasmic division includes nuclear division. In bacteria (Bisset 1952), the nucleus is of a chromosomal shape and has divided completely before polar separation occurs. This clearly indicates that in bacterial division the polar separation is a nuclear distribution process, i.e. the protoplasmic division process. Polar separation

here does not imply nuclear division. It appears, from this comparison of mitosis with bacterial division, that each anaphasic chromosome set in mitosis is the nucleus. And, in mitotic cells the nucleus is of vesicular shape. It suggests that something that may throw a light on the present question may be present in this vesicular form of the nucleus. Thus, in order to obtain a clearer idea of the nature of mitosis, the morphological relation between the vesicular-shaped nucleus and the chromosomes which are a similar shape to the bacterial nucleus must be compared. For this, perhaps, our first task would be to study the structure of chromosomes. It is of course needless to recapitulate the whole history of this investigation here, but since the question has an intimate relation with the present investigation, a brief sketch of this is

given below.In the early days of work on the chromonema structure of chromosomes,

there were disputes over the number of chromonemata1 in the chromosome. One author thought there was only one, while others thought there were two or four. While the majority of authors supported the two-chromonema theory, Nebel repeatedly found four, and later, the present author, who had previously advocated the two-chromonema theory, decided, in view of his own investigations, that Nebel's view was the most plausible. Results of recent investigations using the electron-microscope also favour the idea of a larger number of chromonemata. As well as the recent work of Nebel, we may mention here in particular the work of Yasuzumi in animals and of Shigenaga in plants, because the present author was kindly allowed to examine high powered enlargements of their photomicrographs and lantern slides. We could count 8 thread-shaped structures (the "chromosomal microfibrillae" of Yasuzumi) in some species, 16 in some, and still more in other species. Nebel (1957) has recently counted as many as 64. He says: "This observation implies that a

1 We call the thready structures in the chromosome chromonemata and chromonemata

invested with a matrix substance are called chromosome. We assume that the primary chromosome contains only one pair of chromonemata. When each chromonema is doubled by formation of 2 new chromonemata, a system of 4 chromonemata is established (see later) in the chromosome, and 2 new pairs are dispaired along the original line of pairing. After dispairing, each new pair becomes invested in a matrix, and develops into a new chromosome. The two pairs may, after this development, lie in the mother chromosome. Thus, the mother chromosome now contains 2 new chromosomes. Each of these daughter chromosomes contains 2 chromonemata. Hence the mother chromosome contains in all 4 chromonemata. If the

mother chromosome does not split into 2 daughter chromosomes, and if new formation takes

place again in each daughter chromosome, the mother chromosome will come to contain in all 8 chromonemata, and so on. In our model, therefore, the number of chromonemata in the chromosome depends on the number of repeated new formations before chromosome splitting. We call this chromosome structure a chromosomes-within chromosome structure, or simply

a structure in a nest.

1963 A Model-building Study of Mitosis 215

given locus probably consists of about 64 identical units. Each of the 64 units may be compound, but their ultimate structures are beyond the range of present

methods."

Taylor and others (1957) have recently obtained results by autoradiography which indicate, according to Shinke (1958), that there are two chromonemata. However, the reasons why so many threads can be seen, and why their number is always a definite multiple of two, indicating their common origin, remains unknown. At present, there seems to be no way of reconciling the different findings in this perplexing problem. Yet, a solution would probably constitute a new landmark in the advancement of our knowledge on chromosome structure, and the day may even come when the two-stranded theory is generally accepted as representing the typical structure. Nevertheless, we still consider that there might be cases with more than two chromonemata.

Starting from this view point of recognizing the multi-stranded structure of chromosomes as a possibility, we will attempt first to build a model nucleus with its probable history (paper III), and then to develop our idea to the building of a model mitosis (paper V) and a model meiosis (paper VII). By modelbuilding, of course, we do not mean the superficial copying of natural phenomena by mimicking, but the analysis of the different forms of a phenomenon from one point of view, and the reconstruction or model-building of each form, made so that all forms of the phenomenon can be reduced to one single system based on a definite principle. In the present study, we intend to build models of division in which respective types can be explained as a dispairing phenomenon taking place under the control of a definite principle. We assume that dis

pairing takes place when the original pair which is to be dispaired, is transformed into the form of "a pair of pairs" (system of 4) by a second pairing. While in the majority of cases this dispairing system of 4 is of simple form and homogeneous structure, i.e. a form in which the 4 components of the system are identical, in the cases of protoplasmic division and cell division in mitosis, the systems are of heterogeneous structure. They are synthetic forms or figuratively speaking, hybrid forms, because in these cases the nucleus and cytoplasm have to be taken into account. Here, the two components are assumed to play their respective roles in different capacities according to the type of division with which they are concerned. In the case of protoplasmic division, where the two components are to be paired in their own capacities as nucleus and cytoplasm respectively, the model is built with reference to their

division centers. In the case of cell division, where, if division is a dispairing

phenomenon, the cell should be a double protoplast structure (bacterial cell, see later) in contrast to what is observed, the model is built on the assumption that here, the two components represent together a double structure and play roles similar to that of the pair of protoplasts in the bacterial cell. We will consider later the basis on which this assumption might be useful. In a model thus built, and in reference to its predominant feature, i.e. the part which is


conspicuous on bi-polar separation1, mitosis is a protoplasmic division rather than a nuclear division, although it is, strictly speaking, a composite division also including the cell dispairing process. We understand that in bacterial division these component divisions are each an independent system of homogeneous structures2, except in the case of protoplasm division.

If mitosis and meiosis as a double mitosis are dispairing phenomena following the law that dispairing takes place whenever a system of 4, consisting of two pairs, typically of successive origins, is formed, we shall be ready to give an answer to the question of "why two meiotic divisions should take place instead of one". This question is one on which Wilson (1925) repeatedly laid emphasis in his book, "The Cell" (paper VII).

From the present study, we have the following impressions about division, especially with reference to evolution. The protoplasm has a strong tendency to unify. It is a whole and balanced system. The system may be agitated by both internal and external causes. Thereon, the tendency to unify appears as a form of regulation. The regulation may have a protective meaning. Under certain unfavourable conditions in vegetative activity, it may give rise to a new

formation (cf. Fujii's theory of mitosis, 1958) that will cause pairing with existing or older structures with which the new formation is related. If in this case the pair is transformed into the form of a pair of pairs (system of 4) by second new formations, a regulation takes place which we assume as due to a principle that controls the paired state, so as to keep it unchanged by dispairing

the original pair (paper II). If the protoplasm were devoid of this regulatory ability, no regular unit cell could exist, nor would the cell have any regular division permitting its survival. Division should have a protective meaning in a broad sense.

We assume that the simplest form of dispairing operating under the control of the principle of pairing was the original type, and that this is the basic form, of all divisions in modern forms (papers IV and V). This dispairing

phenomenon constitutes the law of dispairing. The dispairing of this form results in the formation of the structures of duplex constitution. An example is found in bacterial cell division.

In bacteria (Bisset 1952), the cell appears at first sight to consist of a single protoplast, but it contains 2 nuclei of chromosomal shape3 at each pole, and in all 4. According to Bisset's diagram, the 2 nuclei at each pole separate to the poles in the ensuing division, and the cell divides, so as to produce 2 daughter cells, each containing 2 nuclei, one at each pole. Each of these nuclei

1 In our model , separation into two separate duplicates is distinguished from the central feature of division by the name "duplication". We assume that division constitutes predominantly dispairing, a phenomenon by which the paired relation is lost (paper II). The polar separation movements and so-called cell division are included in the duplication process.

2 Although cytoplasmic division must be very complex , we understand that it is essentially a dispairing of a homogeneous type with identical components (paper IV).

3 According to Bisset's review on bacterial cytology , in certain species the nucleus is of a vesicular instead of a chromosomal shape. We will consider the division of these cells in paper VI.


soon divides giving rise to the vegetative form of the cell containing 2 nuclei

at each pole. This bacterial division may be explained as follows:-The

bi-polar separation of the nuclei should mean a protoplasmic division. We

may, therefore, conclude that each daughter cell contains two protoplasts, or

we may say that the bacterial cell is of duplex constitution. Each protoplast

will divide, and the cell will become quadriplex in constitution. This condition

is not permitted by the principle of pairing. Thus dispairing takes place and

two daughter cells are produced, each of duplex constitution. These phenomena

constitute cell division in bacteria. The resulting cells in bacteria are of duplex

constitution and are in the vegetative phase.

Mitosis appeared and reduced this duplex constitution of the pre-mitotic cell to a simpler form. This is a conclusion based on facts. Now, we assume that this mitosis took place when the nucleus, which was of chromosomal shape throughout the cell cycle as in bacteria and of multi-stranded structure as in chromosomes, became vesicular-shaped (paper III and V). This transformation

gave the strands freedom for independent movement and permitted every pair of halves of the strands to become regularly distributed to their legitimate daughter cells. This was accomplished by mitosis and gave ample field for reproducible transmissive gene differentiation. We compare these strands with the chromosomes (paper III).

As we see in bacterial division, cells with chromosome-shaped nuclei divide following the law of dispairing. Cells with a vesicular-shaped nucleus divide by mitosis, and we have certain reasons to conclude that mitosis too, should follow the law of dispairing. In this latter case the cell is of a simple constitution, in contrast to the case in bacteria. While both cells of simplex and duplex constitution can divide, mitotic cells of multiplex constitutions (multinucleate cell) do not. In order for a mitotic cell to divide, it must have a simplex constitution. In polyploid forms the nucleus is multivalent, but in a uni-nucleate condition. Hence the cell is of simplex constitution and can divide. By a simplex constitution of the cell it is implied that the cytoplasm is in a simplex condition. This condition of the cytoplasm is a factor of primary im

portance for regular mitosis (paper IV).The following questions now arise: How was mitosis made to reduce the

duplex constitution of the cell to a simplex one? Why must cells be of a simplex constitution in mitosis before they can divide, while in bacteria those of duplex constitution can divide? It seems that for mitosis there is some condition which replaces the duplex constitution we find in bacterial cells, and we see that the only morphological difference between the two cases is that while in bacterial cells the nucleus is of a chromosomal shape, it is vesicularshaped in mitotic cells. Here, we shape a model assuming that synthetic pairing occurs during pairing. In typical cases, pairing is due to new formation (internal pairing, case of division) or syndesis between pre-existing structures

(external pairing), and in these cases, the structures that pair and form the


dispairing system are identical with each other (homogeneous type). While this is the general rule, the synthetic pairing we propose occurs where the

pairing appears to take place between two differentiated unit structures (heterogeneous type). There are two examples of this. One is the case found in protoplasmic division, and the other is the case in question. In both, division results in a simplex constitution. The protoplasm consists of the nucleus and cytoplasm, and in the division of the protoplasm the two elements should become distributed equally in the daughter forms. For this distribution, we have to assume that pairing takes place between these protoplasmic elements. This assumption contradicts the general rule of pairing. Therefore we assume that the dispairing system is composed of 4 division centers1 of these elements which may be taken as identical. In this form of the dispairing system, pairing is essentially synthetic, but virtually of the internal type, since here the system means a protoplasmic division (papar II). In cell division, pairing is assumed to take place between the cytoplasm and the vesicular-shaped nucleus, both in the capacity of mero-protoplasts, since when there is no transformation of the nuclear shape at any stage in the cell cycle (bacterial case), the cell has a duplex constitution, and when transformation takes place at the end of bi-polar separation (mitotic case), the cell has a simplex constitution. We shall return later to the question of whether there is any support for the assumption that the nucleus and cytoplasm constitute a pair of mero-protoplasts. Briefly, our model explains that the simplex constitution occurs when the duplex constitution is replaced by a structural duplicity (differentiation into cytoplasm and nucleus in the protoplasmic division and 2-mero-protoplast structure in the cell division). The model thus answers the first question raised above, since the protoplasm was considered to be structurally duplex. The model also answers the second question above, since, as the cells are structurally duplex, it is in a condition which corresponds with the genuine duplex condition in bacterial cells as far as division mechanism is concerned, and no more complex structure can form the dispairing system of 4.

Thus, we see that the cell which can divide, and hence which is a unit of

primordial form, is of simplex or of duplex constitution, depending on whether division is mitotic (as in cells with a vesicular shaped nucleus) or pre-mitotic

(as in cells with a chromosomal shaped nucleus). Thus, while the protoplast is a living unit, the cell is another unit, and of higher order, since several

protoplasts may be present in it. The cell should have its own divisional process. Cell division cannot be represented by protoplasmic division. Strictly speak

ing, mitosis is not a mere protoplasmic division, but a composite division of

the whole cell, in which nuclear, cytoplasmic, protoplasmic and cell divisions

take place in cooperation. On the other hand, a cell of multiplex constitution

1 In this model , the nuclear division center is taken as a collective form consisting of the centromeres, and the cytoplasmic division center is also regarded as having a similar collective nature (paper IV).


(multi-nucleate cell or multi-cellular individual) cannot divide. This is a cell in a developed form. It is a vegetative form and exclusively ontogenetic in

character. Diploid and polyploid cells, on the other hand, are primordial forms in which the protoplast grouping or multiplex constitution is replaced by a

multivalent simplex constitution (a case of "disunion", see later).There is another form of regulation for protection. This is the contraction

process, which reduces the surface area. Depending on the colloidal state of the structures involved contraction may or may not appear in the form of pairing. Certain reasons suggest that syndesis was originally a pairing due to contraction for protection (papers II and VI).

Mitosis transformed the earlier cell form of duplex constitution into a new form with a simplex constitution. This transformation prevented the intracellular type of pairing, which we see in the bacterial maturation division

(Bisset 1952). The inter-cellular type of pairing or syndesis, which is regarded as the developed form of this type of pairing, was then the only possible form of pairing. A primitive form of sexuality seems to be developed in this change in cell constitution from duplex to simplex (paper II)1.

Sexuality was realized, and the original double form with duplex constitution was recovered as a diploid, though the diploid was constitutionally simplex. This view that diploidy originated as a substitution for the duplex constitution, implies that the diploid phase is the main phase of generation, while the haploid

phase is a subordinate phase, which is unavoidable in so far as diploidy substitutes for the duplex constitution. The former is not a legitimate generation, but a part of one. We may mention in this connection that gametes are not entities which constitute independent generation, although they are essentially complete protoplasts. There seems to be a restraient. They can develop into a generation only when sexual fusion occurs. This is the rule, though fusion becomes unnecessary if they are released from this restraint under special conditions. These conditions may be external or internal, or by a stimulus acting as a trigger for their release. These facts in connection with the fertilization

process establish the rule and conform that in normal cases the diploid phase is the only phase which constitutes a genaration2.

1 There is the possibility that in bacteria too , the duplex constitution may be transformed into a simplex one at a certain phase in the life cycle, if the nucleus becomes vesicular. Hence there is the second possibility that there may be sexual forms in bacteria, too. This question will be considered in later papers.

2 In plants , the haploid phase appears to constitute a generation. We regard this as an excessive phase occurring as a transitional form in the transformation of the generational form from a duplex to a diploid constitution (animal type). In this transitional phase, the cell called a spore is never neutral in a strict sense, but is latently sexual. The haploid phase can thus be reduced so as to transform the plant type form of generation into the definitive form of the diploid generation, or animal type (Cladophola glomerata, Allen, 1937). This distinction between the forms of generation in plants and animals constitutes a radical difference between these two great groups of living matter. The linear growth in plants (pro-embryo in higher plants) and radial growth in animals in embryonal development is another distinction of equal significance in the sense that the plant type growth involves a longer way to the destination. This is also shown in the plant generation form by the existence of an excessive haploid phase. The author is indebted to Mr. Iijima for suggestions about the l atter case.


Sexuality would not be a phenomenon of incidental occurrence without causal relations, and diploidy may have originated as a substitution for an earlier form of the duplex constitution. In fact, however, in established sexual forms, the duplex constitution is represented only by a nucleus of bivalent constitution. The cytoplasm or another element of the protoplasmic system is not involved in the amphimixis. This is significant in the model-building study of mitosis, which is the division that results in the simplex cell constitution. If the nuclear bivalent consitution is only a substitution for the duplex constitution, the nucleus may have originated in the protoplasm. Therefore the cytoplasm is simply a portion of the protoplasm which is the antithesis of the nucleus, and both are essentially protoplasmic in nature. Therefore, we may assume that these two protoplasmic elements had the same origin and are mero-protoplasts. This relation between the nucleus and cytoplasm may render it possible to assume that, if they are in the same physical state, being of homologous origin, they may pair with each other, provided that diploidy is a substitute for the duplex constitution. We assume this, and in our model-building study we assume that pairing can take place between a vesicular nucleus and cytoplasm forming a synthetic system of 4. Thus we can explain how mitosis came to result in a cell of simplex constitution (paper V). The magnitude of the protoplasmic elements which are assumed to pair

may differ, but we know of heteromorphic pairing in chromosome syndesis. Another significance of the fact that male cytoplasm does not participate in amphimixis is that if it did and caused a duplex constitution, the dispairing system of 4 for regular mitosis would not form in diploid cells. If the double cytoplasm assumes a simplex constitution and because in the life cycle there is no opportunity for its being reduced, it should continue to increase indefinitely. The doubled cytoplasm of the simplex form is not reduced to the single state by meiosis. The doubled cytoplasm in the dulplex form will also complicate regular meiosis, not only at the second division (mitosis), but also of the first division (paper VII).

For nuclear pairing of inter-celluar type, not only the nucleus but also all the protoplasm must be able to pair. This should also be true for nuclear

pairing of the intra-cellular type, though we have no direct evidence on this point. At least for inter-cellular nuclear pairing, we must assume in pairing that all the protoplasm, or syndesis in broad sense, is involved. In this sense of

inter-cellular nuclear pairing, pairing between cells is the first process, and protoplasm fusion, through which nuclear pairing is realized is the second. This

pairing before fusion is essentially an amphimixis. In the present model-building study, therefore, amphimixis is followed sooner or later by nuclear pairing of an inter-cellular nature, i.e. chromosomal syndesis, a pairing which causes a new type of mitosis, viz. meiosis. Thus, the regular recurrence of diploid

generations was firmly established by pairing which caused the generation to be diploid (papers VI and VII). In this model, amphimixis is a pairing rep-


resenting the beginning of syndesis, and meiosis which is initiated by chromosomal syndesis is the division marking its end. Amphimixis and meiosis to

gether constitute a dispairing system to restore the original unpaired cell condition seen before amphimixis. In this cellular dispairing system, the cell pairing occurring at the beginning of amphimixis is the first pairing of the system. This pairing proceeds to fusion, and the pairing lost in fusion is regained at the first division in meiosis. The second divisions in meiosis produce the second pairings of the dispairing system, and the pairing that has been regained from the fused state is dispaired. The second division in meiosis is a mitosis of normal type. Hence the duplication process follows dispairing to give 4 free cells simultaneously.1 Meiosis is the dispairing (disjunction) system not only in the chromosomes or nucleus, but also to the cell, to restore the original unpaired state of the whole cell. In short, in our model, amphimixis and chromosome syndesis constitute a system of inter-cellular nuclear pairing, chromosome syndesis and mitosis constitute meiosis, and amphimixis and meiosis constitute a cell despairing system. The result of this in the sequence of events is the complete recovery of the original unpaired state. Sexuality caused diploidy, and diploidy could be a substitute for the duplex constitution, since the law of dispairing permits the regular recurrence of the diploid state.

We have some reasons to assume that mitosis originated in connection with the transformation of the vegetative form of the nucleus into a vesicle (papers IV and V), and that mitosis provides a clue to the development of the process of intra-cellular pairing into sexuality (paper II). Thus, the appearance of the vesicular-shaped nucleus is an event of special significance in the history of evolution. But, it is to be remembered that the unificatory power of the

protoplasm must always have been in operation during these developments. For instance, chromosome syndesis, which only takes place between homologous chromosomes, is one expression of this tendency to unify. It assures that syndesis occurs between two nuclei carrying individual chromosomes. This is a form of unification in which the syndetic behaviour of the chromosomes corresponds exactly with the same behaviour of the whole nucleus. Bi-polar separation is another expression of unification which is as important as syndesis.

In pre-mitotic bacterial cells too, the nucleus which is chromosomal in shape pairs with another nucleus in the maturation division. These two facts in mitotic and bacterial cells suggest that the chromosomes are products of the splitting (disunion) of the chromosomal-shaped nucleus which may be a compound nest structure, and also that the chromosomes can be of a condensed form, because they are "mero-nuclei" and may retain their ancestral structure at some phase even after being split off. In our model, the nucleus is a col-

1 In rare cases, which are more frequent in animals than in plants, the nucleus in

interkinesis does not assume a vesicular form. In these cases too, normal cell tetrads are

formed, because a normal second division follows which can give rise to a system of 4

protoplasts or cells (comp. the case of bacterial cell division).


lective structure united by the unifying tendency of the protoplasm.1Division and sexuality constitute the primary form of the basic line of

evolution. We take the polyploid cell basic line as a secondary form, and the ontogenetic developments leading to multi-cellular forms as elaborations of these primary and secondary forms of basic line. We consider, therefore, that ployploidization and multicellularization are secondary evolutionary developments, which originated for protective purposes.

Protoplasm has an aggregative tendency. The multi-stranded structure of the chromosomes or the structure which is more adequately called a nest structure, the multi-cellular structure, the multi-nucleate structure, and the poly

ploid structure, all can be regarded as indicating this tendency of the protoplasm. Of these, polyploidization is due, in many cases, to an abbreviation of the

process of mitosis ("restitution" phenomenon) and the other cases are due to the complete absence (the multi-nucleate structure), abbreviation (the multicellular structure) or postponement (chromosome splitting) of post-dispairing separation (duplication)2. All these have their own significance in evolution, but we should like to discuss here the disunion which takes place in the compound nest structure, and which makes this structure significant.

By the disunion of the nest structure, we mean a liberation of the constituent members from structural restraint, so that each member develops a

greater freedom to display its capacity. The development of protoplasm with an aggregative tendency seems to be the object of disunion of this kind (paper IV). It seems that this type of disunion has an important bearing on evolution, in the sense that it involves the transformation of one form into another that will permit further development to the maximum capacity of the original unit. In the disunion which occurs in the transformation of the nuclear form and which gives the genes a wider field of reproducible transmissive differentiation, we see that while the unification is due to structures present before disunion, it is due to behaviour after disunion. Protoplasm compensates for structural disunion by introducing a new relation expressed in behaviour, i.e. syndesis. In our case disunion is not a complete destruction, but a liberation controlled by the principle of unification. We feel that the protoplasm is a dynamic

1 In hybrids , new forms are produced through meiosis. This is a question involving collective structure of the nucleus and a different question from that of nuclear unification. However, it shows how important sexuality and the collective form of the nucleus are in the diverging progresses of evolution.

2 We can find this shifting in a wide field of biology . There are two types, progressive and retardative. Abbreviation is a case of progressive shifting of the termination of a process, and postponement is a case of retardative shifting. Complete loss of a phenomenon would be taken as an extreme example of the latter type. There is another case which may be

called translocation rather than shifting. The transformation in plants of the sex-determining form from the haploid form (X or Y) into the diploid one (XX or XY) is a case of translocation of the sex-determining phase from the haploid phase to the diploid one in the life cycle. This case of translocation may permit the expression in plants of the generation

form of the animal type which we take as the legitimate form.


whole with the ability to control itself. The principle of unification is present

in the protoplasm. Without this principle, evolution as progressive advance

ment would not be possible.

We have pointed out above the importance of division and sexuality in

evolution. Division was the most fundamental and constitutes the basic line of

evolution. It was also subject to evolution, and mitosis developed in connection with the appearance of the vesicular nucleus. These changes favoured

advancement in evolution and allowed genes ample field for reproducible trans

missive differentiation. But, mitosis reduced the resistance of the cell to un

favourable external conditions by transforming the original duplex constitution

into a simplex one. However this transformation resulted in the appearance

of sexuality which restored the original double constitution to the highest degree

possible under the new conditions. Together with mitosis, sexuality also laid the foundations for the regular recurrence of diploid constitution generations

in the new form. During evolution changes may be advantageous or disad

vantageous, but all cooperate towards progressive evolutional development.

During evolution, we see some basic forms which persist and some on

which further development is based. Examples of basic forms are the form of

dispairing which is controlled by the principle of pairing in division of all types

and generation chains in which identical forms with a normal double constitu

tion recur regularly. Examples of forms which have developed during evolution are the diploid constitution from the duplex constitution, inter-cellular type

pairing developed from intra-cellular pairing for sexuality, and the vegetative vesicular form developed from the chromosomal form of the nucleus.

As stated above, the appearance of the nucleus, especially of the vesicularshaped nucleus, was an important event in evolution. It was important because

mitosis developed as a division and sexuality was a form comparable during

pairing with division. In our models, division represents the principle of pairing when pairing is of internal origin, and syndesis or the central feature of sexual

phenomenon represents pairing of external origin. We may even say that the basic pattern of evolutionary development was determined by the principle of

pairing or the law of dispairing. The law of dispairing was known in the expression: "a pair can exist, but a pair of pairs cannot". These two phe

nomena which constitute the law, was of course observed by Newton and

Darlington in studies on the behaviour of chromosomes during formation of

multivalent chromosomes. Darlington was the first to lay special importance

on this, and explain the mechanism of meiosis. In the present study, we use

this law to explain all types of division. In emphasizing the importance of

the principle of pairing, the author wishes to add that the diagrams of ordinary

and maturation divisions in bacteria given by Bisset (1952) were invaluable for

the present study. By taking these diagrams into consideration our model of

mitosis may appear curious, but it must be remembered that the first nucleus

discovered was vesicular-shaped. This led to the idea that the normal nucleus


is vesicular-shaped, and it was thought that it divided by a simple constriction. This idea of the mitotic figure led us to believe that mitosis was another form of nuclear division, and indeed the only form. In these investigations on mitosis, the question of whether mitosis was a nuclear division, was not even considered. It was taken for granted. However we must, without preconceptions, reconside what mitosis is.

The idea of a model-building study came from the following statement by Alexander and Bridges (1928)-"As we follow up into material groupings of

greater and still greater complexity, we observe the development of what to our intelligence seem to be new 'forces' and new 'laws,' although these are no doubt, dependent on and explainable in terms of more complex behavior or relations of the similar underlying particulate units." We thought that there must be a basic form and a definite principle for comparable types of phenomenon.

Our models are artificial and simple, so that the real structure of genes, for instance, is not considered. Thus the present model-building study must not be confused with studies on real phenomena in nature. We do not attempt to explain actual cases. We try to interprete rather than explain. But, it is hoped that these models will be revised in the light of results from _??_ctual investigations, or be re-built by later results, so that the models will some day approach the truth. Thus we hope that our models will help to explain facts by suggesting questions or interpreting results of future studies. It is also hoped that studies on the division of lower organisms and especially various bacteria and Schizophycea will throw light on the problem of the evolution of divisional forms, and explain them better than the present models do. In the present study, publications will be made intermittently. In ensuing papers, models of division in the general sense and some related phenomena (paper II), and models of the vesicular nucleus (paper III), the pre-mitotic protoplasm division (paper IV), mitosis (paper V), the pre-meiotic maturation division (paper VI), and meiosis (paper VII) will be presented.

Literature cited

Alexander, V. and Bridges, C. B. 1928. Colloid Chemistry 2: 9.Allen, C. E. 1937. Am. Nat. 71: 193.Bisset, K. A. 1952. Intern. Rev. Cytology 1: 93.Fujii, T. and Mizuno, T. 1958. J. Fac. Sci. Univ. Tokyo, sec. IV,_??_ part 2: 199.Nebel, B. R. 1957. J. Heredity 48: 51.Shinke, N. 1958. Saibo ("The Cell" in Japanese, a symposium, Kyoto) 2: 62.Taylor, J. H., Woods, P. S. and Hughes, W. K. 1957. Proc. Nat. Acad. Sci. 43: 122.Wilson, E. B. 1925. The Cell. New York.

Cytologia vol. 28 no. 2 (pp. 113-224)Issued June 25, 1963Ausgegeben am 25. Juni 1963Paru le 25 juin 1963

Documents

A Model-building Study of Mitosis, I. Introduction