FCETAL NUTRITION: 



THE PLACENTA 425 

 The same 



allantoic placenta subsequently occupies (see p. 486). 

 occurs in the bat (p. 489). 



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

 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 



y A/l<mt, ,,',/,, in /,,//,, 



Omphcdoidean 



region of 

 t rophosphere 



Decidua 



FIG. 110. Diagram to illustrate the fu-tal membranes of /:'///////.. Ki.<m 

 Hubrecht's "The Placentation of /;'////<'//.< ,<//,//./.<." t/iim: .l<mr. .!//./-. 

 Science, vol. xxx., 1889.) 



of the chorion by a short stalk of mesoderm, in which 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 coslom spreads round the whole circumference of the ovum. The 

 earliest vessels appear on 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 coninrtinff-xta/k (see p. 490), and through them 

 with the vessels of the chorion by a vascular loop, the \////'\ ?nxifi>rini* 



14 A 



