FCETAL NUTRITION: THE PLACENTA 



425 



allantoic placenta subsequently occupies (see p. 486). The same 

 occurs in the bat (p. 489). 



PRIMATES. In monkeys, old- and new-world, there is no decidua 

 reflexa, and a portion of the trophoblast is in contact with the 

 uterine fluids. But even in Selenka's earliest specimens of monkeys 

 and apes, the yolk-sac is a small, closed sac attached to the ventral 

 surface of the embryonic area, and is entirely separated from the 

 trophoblast. The embryonic area is connected with the inner surface 



; , ,, . 1 llantoidean region 



of trophosphere 



Omphcdoidean 



region of 

 trophosphere 



Decidua reflexa 



__ Uterine 

 **"'' lumen 



FIG. 110. Diagram to illustrate the fcetal membranes of Erinacev.s. (From 

 Hubrecht's "The Placentation of Erinacetis europreus" Qtuir. Jovr. Micr. 

 Science, vol. xxx., 1889.) 



of the chorion by a short stalk of mesoderm, in whicli the 

 vessels run. 



In the youngest human ovum yet examined, the yolk-sac is also a 

 small, closed vesicle, separated from the trophoblast by a single thick 

 layer of mesoblast (Fig. 111). The splitting of the mesoblast occurs 

 very early, even before the appearance of the primitive streak, and 

 the ccelom spreads round the whole circumference of the ovum. The 

 earliest vessels appear 011 the under surface of the sac, and gradually 

 extend over its upper pole, until the whole sphere is covered by a 

 vascular network. The vessels are in direct continuity with vessels 

 which develop in the connecting-stalk (see p. 490), and through them 

 with the vessels of the chorion by a vascular loop, the sinus ensiformis 



14 A 



