THE DEVELOPMENT OP VERTEBRATA. 



265 



secondly by the zona radiata, in which all structures derived 

 from the ovum are enclosed. The wall of the yolk-sac, how- 

 ever, becomes fused with the portions of these membranes with 

 which it comes into contact, 

 and for the membrane formed 

 by this fusion a distinct name 

 is desirable : it is called the 

 false chorion (fig. 140). The 

 false chorion comes into con- 

 tact with the wall of the 

 uterus and unites closely with 

 it, forming an attachment of 

 the embryo to the uterus, 

 which may be called the 

 tem.2)oraryor yolk- sac placenta. 

 By means of this connexion "l^S^SST" 



(After Balfour.) 



it seems probable that nutri- 

 tive materials are conveyed 

 for a time from the blood-vessels of the mother into those 

 of the yolk-sac, and so to the embryo. 



7. Allantoic Placenta. But when the allantois grows 

 out, it also fuses with the remainder of the outer membranes, 

 forming the true chorion. This comes in contact with the 

 wall of the uterus and forms a far more important and 

 permanent connexion the allantoic placenta (or placenta 

 simply). The true chorion is a circular disc, hence the 

 rabbit is said to have a discoidal placenta (chap, xii., 9). 

 From the true chorion numerous projections called villi 

 (resembling the villi of the intestine in shape) grow out and 

 become closely interlocked with similar processes of the wall 

 of the uterus. It is to the compound structure thus formed 

 (partly maternal) that the name placenta is given. Thus 

 a very close connexion is established, and by diffusion the 

 blood-plasma in the vessels of the uterus passes into those 

 of the allantois. Here, then, we finally have the clue to 

 the absence of yolk. A better method of nutrition has 

 taken its place -a method that has the advantage of pro- 

 tecting the embryo up to the time of birth. 



So close is the connexion of chorion and uterus that 



