FCETAL NUTRITION: THE PLACENTA 387 



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 ^ it persists till birth, but this is denied 

 by Duval.^ 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 waU 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, hke 

 the Carnivores, a zonary placenta, 

 but Uttle is known regarding the 

 development of the foetal mem- 

 branes. Assheton^ has recently ^^^ 82 -Transverse section 

 given an account of two early through the blastocyst of the 

 embryos of Hyrax. In the younger, 

 the yolk-sac occupied about three- 

 quarters of the surface of the 

 blastocyst, and the allantois the 

 remaining quarter, the ovum pos- 

 sibly being whoUy embedded in 

 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 

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



' Bischoft, Entwickelungsgeachichte der Saugethiere und des Menschen, 

 1842. 



^ Duval, "Le Placenta des Carnassiers, " Jour, de I'Anat. et de la Phys., 

 1893. ^ AsshetoD, Phil. Trans. Roy. Soc, London, loc. cit. 



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.) 

 A., allantois ; A S, splanchno- 

 pleur of allantois ; A.V, allan- 

 toic blood-vessel ; C, coelom ; 

 V, commencing folds from 

 which villi spring ; Y, solid 

 yolk-sac. 



