What happens after fertilization

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4-cell stage

Morula

Blastula

Gastrula

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Aim: What happens after cleavage begins?

Do now: How does a single celled zygote become a gastrula?

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Differentiation (Organogenesis)

Organogenesis is the formation of the organs (Organo = organs, genesis = creation)

Arises from the layering of cells that occurs during gastrula stage

The layers are germ layers; they have specific fates in the developing embryo:

Endoderm

The innermost layer

Goes on to form the gut

Mesoderm

In the middle

Goes on to form the muscles, circulatory system, blood and many different organs

Ectoderm

The outermost

Goes on to form the skin and nervous system

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Late Gastrula

Mesoderm

Endoderm

Ectoderm

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Differentiation of Primary Germ Layers (from the gastrula)

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Early Human Development Summary

Meiosis makes sperm in males and ovum in females

Sperm and ovum unite nuclei to form a zygote

Zygote undergoes cleavage and becomes gastrula with 3 germ layers

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Embryonic Membranes

Protect and nourish the embryo

There are four embryonic membranes

Develop from the germ layers, but are not part of the embryo and are lost at birth

The chorion and amnion enclose the embryo

The chorion surrounds the entire embryo

The amnion encloses the embryo and forms an open volume between the embryo & the amnion called the amniotic cavity

The amniotic cavity fills with amniotic fluid, which envelops the embryo and cushions it

The amniotic fluid can be sampled to test for developmental abnormalities

The allantois is an outgrowth of the gut

In reptiles and birds, it stores nitrogenous wastes

The yolk sac encloses the yolk in vertebrates with yolk-rich eggs

In humans, there is no yolk sac, but the yolk aids in formation of red blood cells

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Extraembryonic Membranes

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Human Prenatal Development

Gestation lasts 266 days from fertilization to birth

Development begins in the oviduct

About 24 hours after fertilization, the zygote has divided to form a 2-celled embryo

The embryo passes down the oviduct by cilia and peristalsis