434 AN INTRODUCTION TO ZOOLOGY 



more external and extra-embryonic were going on. These lead to 

 the formation of the characteristic membranes enclosing the embryo 

 and its attachment to the uterine wall, and we must now consider 

 them. The membranes are on the whole closely similar to those in 

 the chick in origin, but have a quite different fate. The amnion 

 loses most of its value as a protection, the yolk sac is practically 

 a vestigial remnant, the allantois loses most of its respiratory and 

 excretory functions, being mainly concerned with bringing the 

 embryo into relation with its food supply, and the chorion either 

 wholly or in part becomes the chief organ of nutrition and excretion. 

 There is considerable diversity in the parts played by the various 

 structures in different groups of mammals. 



By the time the ovum reaches it the mucous membrane lining 

 the wall of the uterus has become enlarged, highly vascularised, and 

 thrown into a series of folds, and the trophoblastic wall of the vesicle 

 is soon brought into contact with it. In the rabbit and certain 

 other forms it becomes attached to the wall of the main cavity of the 

 uterus, a condition known as central implantation. In man and other 

 species, however, the vesicle bores its way through the superficial 

 layers of the mucous membrane, which closes over behind it, and so 

 it becomes buried, a condition known as interstitial implantation. 

 In the rabbit a horse-shoe-shaped area of the trophoblast, behind and 

 lateral to the embryonic rudiment, becomes modified to form a layer 

 of enlarged actively growing cells, termed the trophoderm, and this is 

 responsible for its attachment to the mucosa. In man this change 

 takes place over the whole surface of the vesicle. 



Amnion and Chorion. 



The processes resulting in the formation of the Amnion and the 

 Chorion in the rabbit closely resemble those in the chick. The wing- 

 like extension of the mesoderm from the axial line leads to the 

 formation of quite a large proamniotic area. The first of the folds 

 to appear is the tail fold, and while this is in progress the head bends 

 down and pushes its way into the proamniotic area, whose anterior 

 margin soon rises up into a fold. Lastly, the lateral folds appear 

 joining the preceding ones, and they all grow upwards to fuse over 

 the top of the embryo. The fusion is more complete than in the 

 chick, and so only a small sero-amniotic knot is left. The separation 

 of the walls of the folds then leads to the formation of (a) the amnion 

 immediately above the embryo, and (b) the chorion in close contact 

 with the trophoblast. Within the former is the amniotic cavity, 

 and between it and the chorion is the extension of the extra- 

 embryonic ccelom. The relations of ectoderm and mesoderm in 

 these two folds is just as in the chick. The mesoderm later invades 



