8o 



CHORDATE ANATOMY 



take form, a constriction involving both somatopleure and splanchno- 

 pleure (Figs. 72, 74) appears between the yolk-sac and the remainder 

 of the embryo. The constriction deepens until the embryo presents the 

 appearance of a small animal having a narrow-necked globular sac sus- 

 pended from the under side of the body (Figs. 72Z), 73). In amniotes the 

 amnion is concerned in this constriction (Fig. 74). As the embryo 

 increases in size the shrinking yolk-sac is drawn up into the body. The 

 inner wall (splanchnopleure) 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 at the time of hatching the somatopleure is ruptured 



Fig. 73.- 



-Young dogfish shortly before birth. The yolk-sac, containing a remnant 

 of the yolk of the egg, protrvides from the ventral body-wall. 



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 description (pages 33-36) of the reproductive arrangements in 

 vertebrates a general account of the embryonic membranes, amnion, 

 chorion and allantois, of reptiles, birds and mammals was given. The 

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

 the embryonic body now makes it possible to appreciate the manner of 

 formation of these membranes in terms of germ layers. 



All eggs are invested by protective coverings which are either pro- 

 duced by the ovum itself or are secreted about the egg by the oviduct. 

 Such membranes consist of material which is not cellular and not in any 

 sense living. They have merely passive functions. The amnion, 

 chorion and allantois are produced by the germ layers at a relatively 

 advanced stage of the embryo. They are constituted of living cellular 

 material and they are actively concerned with such important functions 

 as nutrition, respiration, excretion and circulation. 



