426 THE MAINTENANCE OF SPECIES 



superficial ectoderm and the outer or somatic mesoderm are con- 

 tiguous, and together are called the somatopleure. They grow up 

 from the surface to produce folds known respectively as head, tail, and 

 lateral folds, and these folds in turn grow up and over the embryo from 

 the head posteriorly until they meet and fuse. Upon dissolution 

 of the wall at the point where these folds meet, two new complete 

 layers covering the embryo are produced, the inner layer of somato- 

 pleure being known as the amnion, and the outer as the chorion. The 

 amniotic cavity between the amnion and the embryo is lined with 

 ectoderm and becomes filled with a shock-absorbing amniotic fluid 

 which serves the additional function of keeping the embryo moist. 

 Outside the amnion is the chorionic cavity which is lined with somatic 

 mesoderm. 



All of the time that the head, tail, and lateral folds of the amnion 

 are developing, the yolk is being reduced and the splanchnopleure, 

 composed of the endoderm and splanchnic mesoderm, is growing down 

 and around it to complete the yolk sac. The outer margins of the 

 somatopleure at the base of the developing amniotic folds likewise 

 continue to grow down and around the yolk sac until they finally meet 

 ventrally. This new layer may really be called a continuation of 

 the chorion, while the cavity lying between the outer surface of the 

 yolk sac and the inner side of the chorion is in reality but a continu- 

 ation of the body, or coelomic cavity. Because of its position this 

 portion of the coelomic cavity becomes known as the extra-embryonic 

 coelom. It will be seen from the figure (page 425) that the chorionic 

 cavity is nothing but an outgrowth from this. 



Allaniois 



A yolk sac is developed in all of the egg-laying types of reptiles and 

 birds. Even in the mammals, it is present in a reduced form. Rep- 

 tiles, birds, and mammals, however, develop a fourth embryonic struc- 

 ture called the allantois, which serves as an excretory and respiratory 

 organ. While the yolk sac is attached by a yolk-stalk to the mid- 

 gut region, the allantois develops as a diverticulum from the ventral 

 surface of the hind-gut. Its growth does not start until after the 

 amnion and chorion are in the process of formation. Almost at once, 

 however, this out-pocketing encounters the inner layer of mesoderm 

 so that the allantois comes to be lined by endoderm on the inside and 

 covered by splanchnic mesoderm on the outside. The outgrowth 

 continues, extending out into the extra-embryonic coelom and up 



