FCETAL NUTRITION: THE PLACENTA 



423 



the yolk-villi. Then the yolk-sac becomes less important. The 

 circulation in the decidua reflexa, which surrounds it, decreases and 

 ceases altogether on the sixteenth day, and the wall of the yolk-sac 

 becomes thin and bloodless. " At the same time numerous diverticula 

 grow out from the entodermal sinus into the hilum of the allantoic 

 placenta, and these may still absorb nutriment though they are more 

 probably excretory." Later the outer wall of the invaginated yolk- 

 sac undergoes atrophy and completely disappears. The remains of 

 the yolk-sac cavity are in 

 this way bathed in the uterine 

 fluids. At the same time the 

 villi of the inner wall increase 

 in size and complexity, but 

 whether they absorb the fluids 

 or are entirely excretory is 

 uncertain. 



In the spiny mouse 

 (Acomys caharinus), Assheton l 

 found in a well-advanced 

 pregnancy that the yolk-sac 

 was still extremely vascular, 

 and covered with a columnar- 

 celled epithelium which was 

 much folded. The blood- 



A 



vessels lay in the folds, and 

 so approached closely to the 

 placenta. The yolk-sac was 

 firmly attached to the 

 placenta over the peripheral 

 area, but not so closely as 

 described above for the rat 



FIG. 109. Diagram of the blastodermic 

 vesicle of the rabbit in longitudinal 

 section. (From Hertwig's Entwick- 

 IwngsgescJiichte des Menschen und der 

 Wirbelthiere.} 



e, Embryo ; a, amnion ; a^, allantois with 

 blood-vessels ; f.d, vascular layer of 

 mushroom-shaped yolk-sac ; d.s, cavity 

 of yolk-sac; s.t, sinus terminalis; y, 

 large space filled with fluid. 



and common mouse. In the 



spiny mouse the folds do not become involved in the placental 



tissues. 



The preplacental blastocyst in the beaver has been investigated 

 by Willey 2 who states that its most conspicuous feature is the 

 presence of a deep tract of mesoblastic origin along the length of 

 its embryonic side. This is comparable to the " Haftstiel " of Tarsius, 

 monkeys and man (see pp. 490 and 500). This structure continues 

 to grow in later stages, and by accession of material from the uterine 

 mucosa becomes converted into the definitive umbilico-uterine con- 



1 Assheton, " On the Foatus and Placenta of the Spiny Mouse," Proc. Zool. 

 Soc., London, 1905, vol. ii. 



2 Willey, " The Blastocyst and Placenta of the Beaver," Quar. Jour. Micr. 



-<>, vol. lx., 1914. 



