REPRODUCTION 11,3 



small animal having a narrow-necked globular sac suspended from the 

 under side of the body (Fig. 83). In amniotes the amnion is concerned in 

 this constriction (Fig. 84). As the embryo increases in size the shrinking 

 yolk-sac is drawn up into the body. The inner wall (splanchnopleurej 

 of the sac finally constitutes a small region of the wall of the intestine. 

 In elasmobranchs the somatopleure of the yolk-sac finally flattens out and 

 persists as a part of the abdominal wall. In reptiles and birds the corre- 

 sponding region of the somatopleure is concerned in the formation of the 

 amnion and chorion. At the time of hatching the somatopleure is rup- 

 tured at the constriction between the definitive body and the extra- 

 embryonic structures and everything external to the rupture is abandoned. 



Embryonic and Fetal Membranes 



In the general account (pages 51-55) of the reproductive arrangements 

 in vertebrates mention was made of the embryonic membranes, amnion, 

 chorion and allantois, which occur in reptiles, birds and mammals. The 

 foregoing account of the origin of the germ layers and the shaping up of 

 the embryonic body now makes possible more explicit statements con- 

 cerning the manner of formation of the embryonic membranes. 



All eggs are invested by protective membranes or coverings which are 

 either produced by the egg-cell itself or are deposited about the egg by 

 secretory activity of the walls of the oviduct. Such membranes consist 

 of material which is not cellular and not in any^ sense living. They have 

 merely passive functions. The unique thing about these other mem- 

 branes, the amnion, chorion and allantois, is that they are produced by 

 the germ layers at a relatively advanced stage of the embryo, they are 

 therefore constituted of living cellular material and they are actively 

 concerned with such important functions as nutrition, respiration, excre- 

 tion and circulation. 



The amnion and chorion of reptiles and birds are simultaneously 

 produced by an up-rising fold of the embryonic body wall or somato- 

 pleure (Fig. 84), A chick embryo in the second day of incubation shows 

 a crescentic transverse fold of somatopleure just beyond the head (Fig. 

 85). As development goes on, the head grows forward into the fold 

 and at the same time the fold grows backward over the head. Meanwhile 

 the fold extends backward along either side of the embryo. On the third 

 day a similar fold arises just beyond the tail and begins to grow forward 

 over the caudal part of the embryo. Head-fold and tail-fold having 

 become joined by lateral folds, the embryo is then surrounded by one 

 continuous fold which grows in centripetally from all directions and 

 finally completely encloses the embryo. Where opposite edges of the 

 fold meet above the embryo they become coalesced. Reference to 



