78 



THE EARLY DEVELOPMENT OF MAMMALS. 



that the exact steps of the process are considerably modified in many mammals, 

 in connection with the early modifications which the ovum undergoes in order 

 to secure its attachment to the walls of the uterus (see the section on the 

 trophoblast). The somatopleure forms two folds, one on each side of the 

 embryo. These folds arch up over the back of the embryo. The inner leaf or 

 part of each fold is the anlage of the amnion, Am. It consists of a layer of ecto- 

 derm next to the embryo, and a layer of mesoderm, represented by the dotted 

 line, turned away from the embryo. The remaining portion of the extra- 

 embryonic somatopleure, Cho, extends around both the amnion and yolk-sac, 

 forming a membrane called the chorion, which likewise consists, of course, of 

 ectoderm, which, however, faces away from the embryo, and of mesoderm (dotted 



Sp] 



Som 



Spl 



Som 



Cho. 



FIG. 31. GENERALIZED DIAGRAM OF AN AMNIOTE VERTEBRATE EMBRYO. 



The first figure shows the condition before, the second after, the separation of the amnion from the chorion. 

 Am, Amnion. Cho, Chorion. Cce, Coelom. In, Intestinal canal. Som, Somatopleure. Spl, Splanch- 

 nopleure. Yolk, Yolk-sac. 



line), which is turned toward the embryo. As regards the embryo, therefore, 

 the position of the two germ-layers in the amnion is reversed in the chorion. 

 The two folds continue to grow until they meet above the back of the embryo 

 and unite. The amnion (Fig. 31, Am) has thus become the closed membrane 

 surrounding the embryo, and the chorion, Cho, has become a closed membrane 

 surrounding the amnion, the embryo, and the yolk-sac. 



By the processes indicated we have produced an embryo with its three 

 primary appendages the chorion, amnion, and yolk-sac. To these there is to 

 be added a fourth appendage, the allantois, which also begins its development 

 very early, and arises as a hollow outgrowth from the under side of the caudal end 



