PREGNANCY 63 



the union is complete the outer and inner layers (each of which 

 is composed of ectoderm and somatopleur) are separated by 

 the coelom. The outer layer becomes the chorion, as already 

 described, while the inner layer is the amnion, within which is 

 the amniotic cavity containing the embryo. In man, however, 



'-''^tfBfcr^X — r Ectodermal part ~J ^ ^^"'*^ 



^Hf ^ \ inner cell f J ., ' . 



Magma reticule or ■ — ^ \-i 



jirimitive mesoderm ^^^^^T I 



Amniottc cantii 



Trophohlast 



f— »- A rche nteron 



Fici. 37. — Diagram showing the first difierentiation of the formative 

 cell mass. (From Gray's Aiuttomi/.) 



Fig. 38. — Diagram showing the early stages in the formation of the 

 amnion and archenteron. (From Gray\^ Anatoimj.) 



g 



Amniotic cavity 



— Secondary mesoderm 



— Archenteron 



.'Extra- embryonic 

 coelom 



Fig. 39. — Diagram showing the commencing formation of the 

 extra-embryonic coelom. (From G'n/j/s A/uttom//.) 



and certain other animals this cavity develops from its inception 

 as a closed sac in the middle of the ectodermal part of the inner 

 cell mass. 



Development of the Embryo 



The embryo develops from the layer of ectoderm cells lining 

 the amniotic cavity and forming its floor and from the endoderm 

 cells of the yolk sac lying immediately below the embryonic 

 ectoderm. This area of cell proliferation is known as the 

 embryonic area. The formation of the mesoderm has already 

 been referred to. During the first three weeks of its existence 

 the embryo resembles a flattened disc lying upon the yolk sac. 

 Looked at from above, the area of proliferation appears shaded, 

 the shading being due to the increase of cells, the three germinal 

 layers of embryonic ectoderm, mesoderm, and endoderm being 



