666 



embryology: mammals 



blastocyst (Fig. 520). As it grows, its trophoblast cells stretch 

 and become thinner and flatter (Fig. 518, 7, tr.). Meanwhile the 

 inner mass cells begin to differentiate into two layers, an outer, 

 columnar layer of ectoderm, and an inner, flattened layer of 

 endoderm. The endoderm starts to grow round the blastocyst, 



f.a 



e a 



.bhy 



B A 



Fig. 520. — The blastocyst of a rabbit on the seventh day. — After Kolliker. 



A, From the side ; B, from above. 

 b.hy., Boundary of the endoderm ; e.a., embryonic plate. 



lining the trophoblast beyond the embryonic ectoderm. Over the 

 latter, the trophoblast cells (here known as cells of Rauber) 

 become separated and disappear, leaving bare the embryonic 

 ectoderm, which at its edges becomes continuous with the tropho- 

 blast, so that the vesicle remains unbroken. The blastocyst is now 



rues. 



Fig. 521. — A transverse section through the primitive streak of a rabbit. 

 ep.. Ectoderm ; hyp., endoderm ; mes., mesoderm ; pr.gr,, primitive groove. 



practically in the condition of the early blastoderm of the chick, 

 though instead of the immense mass of yolk of the bird's e,^^ there 

 is only the fluid of the blastocyst, and the ectoderm (including the 

 trophoblast) already forms a complete vesicle. In the embryonic 

 plate a primitive streak and groove (where cells immigrate from 

 presumptive areas of the surface), medullary folds, mesoderm, 

 head and tail folds (separating embryo from yolk sac), amnion 

 and allantois now arise in succession (Fig. 522). The mesoderm, 



