DEVELOPMENT OF THE CHICK 123 



splanchnic layer. The somatic layer becomes 

 closely associated with the ectoderm and thus 

 forms the so7nato'pleure, while the splanchnic 

 layer similarly unites with the entoderm to 

 form the splanchnopleure (Fig. 59). The space 

 between somatopleure and splanchnopleure is 

 the body-cavity or ccelome. 



Between the somites and the lateral plate 

 is a narrow zone of mesoblastic cells forming 

 the nephrotome or intermediate cell-mass from 

 which urinary and reproductive organs develop. 



By the extension of the mesoblast round 

 the yolk, and as a consequence of the splitting 

 of the lateral plate of the mesoblast, it follows 

 that ultimately the splanchnopleure immedi- 

 ately over the yolk is separated by ccelome 

 from the somatopleure and becomes the wall 

 of a sac containing the yolk and therefore 

 known as the yolk-sac. Associated with this 

 is the gradual accentuation of the head-, tail-, 

 and lateral-folds, which result in a tucking-in 

 of the body-wall underneath the embryo. In 

 this way the opening on the under surface of 

 the embryo is gradually reduced and the 

 embryo is connected by a comparatively narrow 

 stalk with the extra-embryonic structures de- 

 rived from the rest of the blastoderm. From 



