424 OUTLINES OF CHORDATE DEVELOPMENT 



tive or resorptive surface, taking in materials from the maternal 

 tissues and blood. 



The extent to which the trophoderm erodes the uterine 

 lining varies greatly. Of course where no trophoderm is 

 differentiated, little or no actual erosion occurs. And when 

 the trophoderm is present, the erosion may affect only the 

 epithelium of the mucosa, or it may involve the connective- 

 tissue elements, or even the walls of the uterine blood-vessels. 

 The degree of erosion has been suggested as a basis for the 

 classification of the types of placentae (Grosser), but this and 

 many other facts regarding the later history of the trophoderm 

 are better considered later, in connection with the placenta 

 itself. 



2. The Amnion and Chorion 



Our description of the formation of these membranes may 

 be very brief on account of their general similarity to those 

 of the chick fully described in an earlier chapter (Fig. 178). 



We must distinguish, at the very outset, between two 

 general types of amnion formation found among the Mammals, 

 a distinction that has already been noted above in describing 

 the formation of the embryonic disc and its relation to the 

 tropho blast. In the Carnivors, Ruminants, many Insectivors 

 and some Rodents, such as the rabbit, the amnion is formed 

 from a series of folds of the extra-embryonic somatopleure 

 (wall of the blastodermic vesicle) much as in the chick. In 

 other forms, such as the mouse, guinea-pig, bat, some Insecti- 

 vors, and many Primates, including man, the amnionic cavity 

 arises in situ (entypy), above the embryonic disc, through a 

 splitting of the ectoderm or through the confluence of gradually 

 enlarging spaces (Figs. 152, 153, 155). 



The amnion and chorion of the rabbit may be described as a 

 fair representative of the first type. Here, as in the chick, the 

 mesoderm very early extends posteriorly and laterally from 

 the embryo, but immediately anterior to it the wall of the 

 blastodermic vesicle remains for a considerable period devoid 



