354 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 



close, and the placenta is truly deciduata The cavity of the 

 allantols persists till birth. Between the yolk-sac, the allantois, 

 and the embryo, there is left a large cavity filled with an albumi- 

 nous fluid. 



Fig. 116. 



DlAGRAMMATIO LONGITUDINAL SECTION OF A RaBBIT'S OvUM 



AT AN Advanced Stage of Pregnancy. (From KolUker 

 after Bisohoff.) 



e. embryo ; a. amnion ; a. urachus ; al. allantois with blood- 

 vessels ; sh. sub-zonal membrane ; pi. placental villi ; fd. 

 vascular layer of yolk-sac; ed. hypoblastic layer of yolk- 

 sac ; ed'. inner portion of hypoblast, and ed". outer portion 

 of hypoblast lining the compressed cavity of the yolk-sac ; 

 ds. cavity of yolk-sac ; St. sinus terminalis ; r. space filled 

 with fluid between the amnion, the allantois and the yolk- 

 sac. 



The metadiscoidal type of placenta is found in 

 Man and the Apes. The placenta of Man may be con- 

 veniently taken as an example of this type. 



