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-Formation of germ layer.

-A single layered blastula becomes a trilaminar structure with an outher ectodermal, middle mesodermal and innder endodermal layer.

-Through cell migration from surface of blastula to its interior

What does the ectodermal layer become?
Epidermis of the skin and neural tissue
What does the endodermal layer become?
lining of gastrointestinal and respiratory tract
What does the mesodermal layer become?
Urogenital, circulatory and supportive muscular and skeletal systems
Gatrulation in Primitive Chordates (Aphioxus/ Lancelet)
-Blastoderm at vegetal pole flattens and invaginates
-Formation of a caity= primitive gut/archenteron
-Opening of primitive gut=blastopore
-Endodermal and mesodermal structures relocate from the surface of the embryo to interior, forming a trilaminar embryo=gastrula

Gastrulation in Amphibians
-Invagination cannot occur (yolk-filled cells in veg. hemisphere)
-Cells from surface ( cross point of veg.and a. hem.) move to interior, cleft, pre primitive gut.
-cleft deepens, bc/of inflow of endodermal cells (below) and mesodermal (above=
-circular blastopore formed
-blastocoele destroyed, yolk laden cells at veg.pole move to inner and trilaminar embryo is formed

Gastrulation in mammals
-closely resembles that of birds
-Blastocyst cavity=empty yolk-sac cavity
-Blastocyst= formation of a layer of flat cells derived from embr. disc (lower surface)
- Cells of hypoblast extends and line the blastocyst cavity, forming a bilaminar yolk sac.
- Dev. of germ layers, formation of p. streak and node. Streak acts as initiation site for gastrulation. Adjacent epiblast cells lose ability to respond->only 1 p.streak formed

Left/right symmetry
-Organs, vertebrates, asymmetrically in thoracic and abdominal cavities
-regulated by molecule processes
-pattern dev. during gastrulation

-what it is
-two types

-2 ind. develop in 1pregnancy in an animal (monotocous)

-Dizygotic: two ova, fert. by separate spermatozoa, single breeding cycle
-Monozygotic: single ovum, fert. by 1 spermatozoon. 3 diff. ways:

1. Two-blastomere stage: each b. give rise to separate ind. 30% humans.
2. Inner cell mass stage: duplication of inner cell mass. Dev. with separate amnion, but share yolk sac and chorion. 70% humans.
3. Embryonic disc stage: formation of 2p.streak, each giving rise to separate ind. Share amnion, yolk sac and chorion. 1% human

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