EMBRYO OF TWENTY-SEVEN SEGMENTS 65 



gut in the midline. Its wall is composed of columnar entodermal cells and it is an out- 

 growth of the entodermal layer. Dorsal to the hind-gut may be seen undifferentiated cells 



Primilive knot 

 Somatic mesoderm, I Ectoderm 



Ccelom. \ / rf^*'"*!^., Somatopleure 



r"S53T5 



Ka!*^ 



Splanchnopleure 



Hind-gut Entoderm 



Fig. 69. — Transverse section passing through the hind-gut of a fifty-hour chick embryo. X 50. 



of the primitive streak continuous dorsad with the ectoderm, ventrad with the entoderm of the 

 hind-gut, and laterally with the mesoderm. 



In the chick embryos which we have studied, there are large areas developed 

 which are extra-embryonic, that is, he outside the embryo. The splanchnopleure 

 of the area vasculosa, for instance, forms the wall of the yolk sac, incomplete in 

 the early stages. The amnion, chorion, and allantois are extra-embryonic mem- 

 branes which make their appearance at the fifty-hour stage. These structures 

 are important in mammahan and human embryos and a description of their 

 further development in the chick, where their structure and mode of develop- 

 ment is primitive, will lead up to the study of mammahan embryos in which the 

 amnion and chorion are precociously developed. 



Amnion and Chorion. — These two membranes are developed in all amniote 

 vertebrates (Reptiles, Birds, and Mammals). They are derived from the extra- 

 ernbrvo nic somatopleure. The amnion is purely a protective structure, but the 

 chorion of mammals has a trophic function, as through it the embryo derives its 

 nourishment from the uterine wall. Fig. 70 A shows the amnion and chorion 

 developing. The head fold of the somatopleure forms first and envelops the head, 

 the tail fold makes its appearance later. The two folds extend laterad, meet 

 and fuse (Fig. 70 B, C). The inner leaf of the folds for ms the amnion., the re- 

 mainde r of the ex tra^g mbryonic somatopleure becomes the chorion . The actual 

 appearance of these structures and their relation to the embryo have been seen in 

 Figs. 63 and 64. The amni on, with its ectodermal layer inside, completely su r- 

 rounds the embryo at' the end of the third day, enclosing a cavity filled with am- 

 n iotic fluid (Fig. 71). In this the embryo floats and is thus protected from injury. 

 The chorion is of Httle importance to the chick. It is at first incomplete, but 

 eventually entirely surrounds the embryo and its other appendages. 



Yolk Sac and Yolk Stalk. — While the amnion and chorion are developing 

 during the second and third day, the embryo grows rapidly. The head- and tail 



