410 



VERTEBRATE ZOOLOGY 



rity in the different mammalian groups. In some species, as in cattle 

 and horses, the young at birth are well advanced and, within a few 



h ta 



vb 



FIG. 216. Diagram of the formation of the embryonic membranes and append- 

 ages of the rabbit. A, at the end of the ninth day; B, early the tenth day; C, at 

 end of tenth day. Ectoderm, black; endoderm, dotted; mesoderm, gray, al, al- 

 lantois; as, allantoic stalk; 6, tail bud; c, heart; d, trophoderm; e, endoderm; 

 ex, exoccelom; /, foregut; h, hind-gut; m, mesoderm; N, central nervous system; 

 p, pericardial cavity; pa, proamnion; s, marginal sinus (sinus terminalis); t, tropho- 

 blast; ta, tail-fold of amnion; v, trophodermal villi; vb, trophoblastic villi; y, cav- 

 ity of yolk-sac; y. s, yolk-stalk. (From Kellicott, after Van Beneden and Julin.) 



