570 



ZOOLOGY 



SECT. 



peri.erid. 



FIG. 1227. 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 ; C, blastodermic vesicle with embryonic 

 cell-mass (emb.) separated from trophoblast (tr.) except on 

 one side ; D, blastodermic vesicle in which peripheral and 

 embryonic portions of endoderm have, become established : 

 the break here represented on each side between the two 

 does not occur. E, stage in which the embryonic ectoderm 

 has broken through the trophoblast and become joined to it 

 peripherally. 



derived the embryonal 

 ectoderm and the 

 entire endoderm of 

 the vesicle. 



The outer layer is 

 apparently the equi- 

 valent 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 embryonic 

 endoder m and is 

 formed by outgrowth 

 from it. The rest of 

 the cell-mass gives 

 rise to the embryonic 

 ectoderm. The part of 

 the trophoblast lying 

 over this embryonic 

 ectoderm, known as 

 the covering layer or 

 Rauber's layer, has a 

 widely different fate in 

 different Eutheria : it 

 may thin out and dis- 

 appear. 



A primitive knot and 

 embryonic shield are 

 formed as in Reptiles. 

 The primitive knot has 

 simply the appear- 

 ance of the somewhat 



