FCETAL NUTRITION: THE PLACENTA 387 



Tr 



AS 



only in the early stages. In the dog the yolk-sac is large and 



extends at first to the end of the citron-shaped ovum (Fig. 97). 



According to Bischoff l it persists till birth, but this is denied 



by Duval. 2 The allantois grows out 



on the dorsal side of the embryo, 



and fuses with the diplo-trophoblast 



over a small discoidal area. Later, 



as the cavity of the allantois en- 



larges, it adheres to the whole of 



the blastocyst wall except the 



poles. Subsequently the zone of 



adhesion is reduced in extent (see 



p. 413). 



PROBOSCIDEA and HYRAX. 

 The elephant and the aberrant 

 genus Hyrax have at full-time, like 

 the Carnivores, a zonary placenta, 

 but little is known regarding the 

 development of the foetal mem- 

 branes. Assheton 3 has recently 

 given an account of two early 

 embryos of Hyrax. In the younger, 

 the yolk-sac occupied about three- 

 quarters of the surface of the 

 blastocyst, and the allantois the 



82 Transverse section 

 through the blastocyst of the 

 sheep at the twenty-fifth day. 

 (From Assheton, "The Mor- 

 phology of the Ungulate 

 Placenta," Phil. Trans. Roy, 

 Soc., London, Ser. B., vol. 

 cxcviii., 1906.) 



toic blood-vessel ; C, ccelom ; 

 V, commencing folds from 

 which villi spring ; Y, solid 

 yolk-sac. 



remaining quarter, the ovum pos- 



sibly being wholly embedded in A - allantois ; A S, .splanchno- 

 * J pleur of allantois ; A.V, allan- 



the uterine mucosa. The yolk-sac 



was covered with a network of 



vessels, and the head of the 



embryo dipped into it. It was 



invested externally with a mass of trophoblastic cells, honey- 



combed with spaces and filled with maternal blood, but. no 



villi were developed. In the second embryo the yolk-sac was 



1 BischofF, Entwickelungsgeschichte der Sdugethiere und des Menschen, 

 1842. 



2 Duval, "Le Placenta des Carnassiers, " Jour, de VAnat. et de la Phys., 

 1893. 3 Assheton, Phil. Trans. Roy. Soc., London, loc. cit. 



