DEVELOPMENT OF OTHER VERTEBRATES 431 



covered by ectoderm, and the endoderm forms the roof of an 

 enteric cavity which is ventral to the embryo, while laterally it is 

 in contact with the yolk. Between the ectoderm and endoderm 

 is the mesoderm. Shortly before the stage of development shown 

 by this figure is reached, the so-called amniotic folds have arisen 

 anteriorly and posteriorly, as the figure shows, and also on either 

 side of the embry-o. Since this folding involves the ectoderm 

 and also the somatic or outer layer of the mesoderm, its cavity 

 is an extension of the coelome and is lined with mesoderm. As 

 development continues, the anterior and posterior amniotic folds 

 approach and fuse (Fig. 227 B) and thus the embry^o becomes 

 enclosed in an inner membrane, the amnion, and an outer mem- 

 brane, the chorion, each composed of ectoderm and somatic meso- 

 derm. The cavity between the embryo and the amnion is lined 

 with ectoderm and is called the amniotic cavity; the one enclosed 

 by the chorion is called the extra-embryonic calome. By the time 

 the amniotic folds have thus fused, there has developed from the 

 posterior end of the enteric cavity an evagination known as the 

 allantois, which extends into the extra-embryonic ccclome. The 

 allantois is, therefore, fined with endoderm and covered externally 

 by the splanchnic layer of the mesoderm. It is homologous with 

 the uninary bladder of the frog, although this organ is not present 

 in the adult bird. As the embryo increases in size (c/. Figs. 224 G 

 and 227 C), the allantois extends farther and farther into the 

 extra-embryonic coelome, and its outer face comes into close 

 contact with the inner surface of the chorion. As the white of the 

 egg becomes reduced, the allantois is closely applied to the inner 

 surface of the chorion, which in turn hes against the shell mem- 

 brane and shell, and thus becomes a respiratory and excretory 

 organ by means of which oxygen and carbon dioxide are received 

 and discharged. At the time of hatching, the remains of these 

 membranes are still seen in contact with the shell membrane. 



By continued growth of the embryo and corresponding reduc- 

 tion in the amount of yolk, the chick embryo gradually assumes its 

 final dimensions (Fig. 224 H and J) . The last remnant of the yolk 

 disappears from \dew within the ventral side of its body shortly 

 before the hatching. In these final stages, the name yolk sac is 

 given to the endoderm and splanchnic mesoderm surrounding the 

 yolk, and the constricted stem by which this is connected v/ith 

 the embryo is called the yolk stalk. 



