REPRODUCTION. 



79 



nent, and extending between the amnion and subzonal mem- 

 brane. 



The formation of the chorion marks an important step in 

 the development of mammals in which it plays an important 

 functional part. It is 

 the result of the fusion 

 of the allantois, which 

 is highly vascular, 

 with the subzonal 

 membrane, the villi of 

 which now become 

 themselves vascular 

 and more complex in 

 other respects. 



An interesting re- 

 semblance to birds has 

 been observed (by Os- 

 born) in the opossum 

 (Fig. 83). When the 

 allantois is small the 

 blastodermic 

 (yelk-sac) has vascular 

 villi, which in all prob- 

 ability not only serve 

 the purpose of attach- 

 ing the embryo to the 

 uterine wall but derive 

 nourishment, not as in 



birds, from the albumen of the ovum, but directly in some way 

 from the uterine wall of the mother. It will be remembered 

 that the opossum ranks low in the mammalian scale, so that this 

 resemblance is the more significant from an evolutionary point 

 of view. 



The term chorion is now restricted to those regions of the 

 subzonal membrane to which either the yelk-sac or the allan- 

 tois is attached. The former zone has been distinguished as the 

 false chorion and the latter as the true chorion. In the rabbit 

 the false chorion is very large (Fig. 79), and the true (placen- 

 tal) chorion very small in comparison, but the reverse is the 

 case in most mammals. It will be noted that in both birds 

 and mammals the allantois is a nutritive organ. Usually 

 the more prominent and persistent the yelk-sac, the less so 



vesicle ■^'^" ^" — Diagrammatic dorsal view of an embryo rab- 

 bit with Its membranes at the stage of nine so- 

 mites (Haddon, after Van Bcneden and Jnlin). 

 al, allantois, showing from behind the tail fold 

 of the embryo; am, anterior border of true am- 

 nion ; a. »j area vascnlosa, the outer border of 

 which indicates the farthest extension of the 

 mesoblast; bl, blastoderm, here consisting only of 

 epiblast and hypoblast; o. m. v, omphalo-mesen- 

 teric or vitelline veins ; «. am, proamnion ; ;;/, 

 non-vascnlar cpiblastic villi of the future placen- 

 ta ; s. t, siinus terminslis. 



