588 



ZOOLOGY 



SECT. 



eel 



mb.encL 

 perv.encl 



perLend 



FIG. 1215. Diagram representing sections of the embryo of a 

 Mammal at successive stages in the segmentation and 

 formation of the layers. A and B, formation of enclosing 

 layer (trophoblast) and inner cell-mass destined to give rise 

 to the embryo ; 6', blastodei mic vesicle with embryonic cell- 

 mass separated from trophoblast except on one side ; D, 

 blastodermic vesicle in which peripheral and embryonal por- 

 tions of endoderm have become established : the break here 

 represented on each side between the two does not occur. 

 E, stage in which the embryonal ectoderm has broken 

 through the trophoblast and become joined to it peripherally. 



liquid, a cavity, which 

 is formed in the 

 interior of the sphere, 

 increases rapidly in 

 size. The stage now 

 reached is called the 

 blastodermic vesicle. 

 During the growth in 

 size of the internal 

 cavity the central mass 

 of cells remains in 

 contact with one side 

 only of the outer 

 layer, where it spreads 

 out as a stratum 

 several cells deep. 

 From it are derived 

 the embryonal ecto- 

 derm and the entire 

 endoderm of the vesi- 

 cle. 



The outer layer is 

 apparently the equiva- 

 lent of the extra- 

 embryonal ectoderm 

 of the Bird and Rep- 

 tile, and has been 

 termed the trophoblast 

 or trophoblastic ecto- 

 derm, because of the 

 part which it plays in 

 the nutrition of the 

 foetus. Immediately 

 beneath it, through- 

 out its extent, a thin 

 layer of flattened cells 

 appears the peri- 

 pheral endoderm this 

 is continuous with a 

 similar layer formed 

 on the inner surface of 

 the embryonic cell- 

 mass the embryonal 

 endoderm. The rest of 

 the cell-mass gives 

 rise to the embryonal 

 ectoderm. The part of 



