FCJiTAL NUTRITION: THE PLACENTA 425 



allantoic placenta subsequently uccupies (me p. 486). The same 

 occurs in the bat (p. 489). 



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

 reflexa, and a portion of the tr(jphol)last is in contact with the 

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

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

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

 trophoblast. The eml)ryonic area is connected with the iimer surface 



,- Allantiiidi'iin rciiinii 

 .'' (if trdiilidxplicre 



Oiiiphahiidean 

 region of 

 trophosphem 



Dceiiliia ri'tf''i'n 



Uti'rine 



Fig. ] 10. — Diagiiim to illustiute the fiftal membranes of Erinweiis. (From 

 Hubi'eeht's "The Placentation of Eriuaneus enropcms," Qiinr. Juiir. Jficr. 

 Science, vo\. xxx., 1889.) 



of the choridu by a short stalk of mesoderm, in wliicli the 

 \'essels run. 



In the 3'oiingest human ovum yet examined, the yolk-sac is also a 

 small, closed vesicle, separated from the tro}iliolilast by a single thick 

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

 very early, even before the appearance of the primiti\'e streak, and 

 the ccelom 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 tlie whole sphere is covered by a 

 vascular network. The vessels are in direct continuity with \'essela 

 which develop in the niiiiirctiiif/-dall- (see p. 490), and thi'ough tliem 

 with tlie vessels of the cliorion by a vascular buip, tlie .sinus eiisiforntis 

 14 A 



