SOMATOPLEURE AND SPLANCHNOPLEURE. 



83 



comes complicated in the amniota by the formation of the amnion itself. We shall 

 consider here only what is looked upon as the primitive method of the production 

 of the amnion, and note only 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 trophoderm). 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, 



Cho 



Cho. 



FIG. 45. 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. Cae, Ccelom. In, Intestinal canal. Som, Somatopleure. Spl, Splanchnopleure. 

 Yolk, Yolk-sac. 



turned away from the embryo. The remaining portion of the extra-embryonic so- 

 matopleure, Cho, extends around both the amnion and yolk-sac, forming a mem- 

 brane called the chorion, which likewise consists, of course, of ectoderm, which, 

 however, faces away from the embryo, and of mesoderm (dotted 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. 45, Am) has thus become a 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 of the embryo 

 and expands into the extra-embryonic ccelom or space between the yolk-sac and 

 the chorion. 



