32-3: Fertilization + Development

Occurs in series of specific, sequential stages

Fertilization + Early Development Fertilization

Fusion of two gametes – sperm + egg

Combines 2 haploid cells = 1 diploid cell Called a zygote

Once a zygote is formed, DNA is replicated and cell division starts

Fertilization + Early Development Rapid cell division is known as cleavage

Cell increases the number of cells (2, 4, 8, etc.) Becomes a giant hollow ball of cells (blastula)

with an empty central cavity (blastocoel)

Gastrulation The blastula starts to collapse inward – called

a blastopore

Gastrulation Process of changing blastula to gastrula

Creating a multilayered embryo Marked by the changes in cell shapes

Let’s look at Gastrulation

Gastrulation (cont.) As the infolding continues, a deep cup-shaped

cavity is formed This is called the archenteron

This cavity will give rise to the gut

Gastrulation (cont.) 3 germ layers emerge

Ectoderm – outer layer of gastrula

EndodermEndoderm – inner layer

Mesoderm – middle layer; between ectoderm + endoderm (in triploblast)

Organogenesis Each of the germ layers formed during

gastrulation develops into certain organs ENDODERMENDODERM

Lining of urinary system, reproductive system, + most of the digestive tract, pancreas, liver, lungs, + gills

ECTODERMECTODERM Outer layer of skin, hair, nails, + nervous system

MESODERMMESODERM Skeleton, muscles, inner layer of skin, circulatory

system, + lining of body cavities

What do the 3 germ layers form?

Patterns of Development Coelom

A body cavity completely lined by mesoderm Can be used to help classify organisms

3 types of Body Cavities Acoelomates

Body cavity is absent Mesoderm connects endodermic gut to outer

covering

3 types of Body Cavities Pseudocoelomates

Means “false” body cavity Mesoderm lines fluid-filled coelom + gut is

suspended in the fluid of the coelom

3 types of Body Cavities Coelomates

Means a “true” body cavity

Mesoderm lines body cavity and surrounds + supports endodermic gut

Forms attachments for organs

2 patterns of developmentProtostomeProtostome DeuterostomeDeuterostome

Blastopore becomes the mouth (“1st mouth”)

Blastopore becomes the anus (“2nd mouth”)

Spiral cleavage Radial cleavage

Determinate cleavage Indeterminate cleavage

Schizocoely – coelom up Enterocoely – coelom down

Annelids, Arthropods, Mollusks

Echinoderms, Chordates