FCETAL NUTRITION: THE PLACENTA 421 



stages. In the dog the yolk-sac is large and extends at first to the 

 end of the citron-shaped ovum (Fig. 123). 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 enlarges, it adheres to the whole of the blastocyst wall 

 except the poles. Subsequently the 

 zone of adhesion is reduced in extent 

 (see p. 444). 



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 

 membranes. Assheton^ 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 remainipg quarter, the 

 ovum possibly being wholly 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, 

 honeycombed with spaces and filled 

 with maternal blood, but no villi were 

 developed. In the second embryo the 

 yolk-sac was much reduced, and was 

 " presumably enveloped by the allantois." 

 It had previously been shown by Turner 

 that the yolk-sac disappeared at an 

 early period. 



Fig. 107. — Transverse section 

 through the blastocyst of 

 the sheep at the twenty- 

 fifth dav. (From Assheton, 

 "The Morphology of the 

 Ungulate Placenta," Phil. 

 Trans. Roy. ■ Soc, London, 

 Ser. B., vol. cxcviii., 1906.) 



A., Allantois ; A.S, splanch- 

 nopleur of allantois ; A.V, 

 allantoic blood-vessel ; 0, 

 coelom; V, commencing folds 

 from which villi spring ; 

 Y, solid yolk-sac. 

 EODENTIA. — In Eodents the condi- 

 tions are entirely different. The mesoblast never extends, in the 

 rabbit, rat, or mouse, completely round the ovum, and the yolk- 

 sac hypoblast remains long in contact with the trophoblast, and 

 carries on the, nutrition of the embryo till the tardily formed 

 allantoic placenta is developed. Eegarding the partial extension 



1 Bischoff, EntwickelwngsgescMchte der Scmgethiere und des Menschen, 1842. 



2 Duval, " Le Placenta des Carnassiers," Jwwr. de I'Anat. et de la Pkys., 

 1893. 



5 Assheton, Pkil. Trans. Roy. Soc, London, loc. cit. 



