32-3: Fertilization + Development Occurs in series of specific, sequential stages

Fertilization + Early Development 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 Endoderm – inner layer Mesoderm – middle layer; between ectoderm + endoderm (in triploblast)

Organogenesis Each of the germ layers formed during gastrulation develops into certain organs ENDODERM Lining of urinary system, reproductive system, + most of the digestive tract, pancreas, liver, lungs, + gills ECTODERM Outer layer of skin, hair, nails, + nervous system MESODERM 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 development Protostome Deuterostome 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