Animal Development

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Mammalian Development

The eggs of placental mammals

Are small yolk and store few nutrients

Exhibit holoblastic cleavage

Show no obvious polarity.

Gastrulation and organogenesis resemble the processes in birds and other reptiles.

Early cleavage is relatively slow in humans and other mammals.

Slide 35

At completion of cleavage, the blastocyst forms.

A group of cells called the inner cell mass develops into the embryo and forms the extraembryonic membranes.

The trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus, and the inner cell mass of the blastocyst forms a flat disk of cells.

As implantation is completed, gastrulation begins.

Slide 36

Early embryonic development of a human

Blastocoel

Trophoblast

Uterus

Endometrial epithelium (uterine lining)

Inner cell mass

Slide 37

Early embryonic development of a human

Trophoblast

Hypoblast

Maternal blood vessel

Expanding region of trophoblast

Epiblast

Slide 38

The epiblast cells invaginate through a primitive streak to form mesoderm and endoderm.

The placenta is formed from the trophoblast, mesodermal cells from the epiblast, and adjacent endometrial tissue.

The placenta allows for the exchange of materials between the mother and embryo.

By the end of gastrulation, the embryonic germ layers have formed. The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way.

Slide 39

Early embryonic development of a human

Yolk sac (from hypoblast)

Hypoblast

Expanding region of trophoblast

Amniotic cavity

Epiblast

Extraembryonic mesoderm cells (from epiblast)

Chorion (from trophoblast)

Slide 40

Early embryonic development of a human

Yolk sac

Mesoderm

Amnion

Chorion

Ectoderm

Extraembryonic mesoderm

Atlantois

Endoderm

Slide 41

Four stages in early embryonic development of a human

Yolk sac

Mesoderm

Amnion

Chorion

Ectoderm

Extraembryonic mesoderm

Trophoblast

Endoderm

Hypoblast

Expanding region of trophoblast

Epiblast

Maternal blood vessel

Allantois

Trophoblast