42 PRELIMINARY ACCOUNT. [CHAP. 



ing the alimentary canal and its tributary viscera, the 

 splanchnopleure \ 



This horizontal splitting of the blastoderm into a 

 somatopleure and a splanchnopleure, which we shall 

 hereafter speak of as the cleavage of the mesoblast, is 

 not confined to the region of the embryo, but gradually 

 extends over the whole of the yolk-sac. Hence in the 

 later days of incubation the yolk-sac comes to have 

 two distinct coats, an inner splanchnopleuric and an 

 outer somatopleuric, separable from each other all 

 over the sac. We have seen that, owing to the 

 manner of its formation, the ' embryonic sac ' is con- 

 nected with the ' yolk-sac ' by a continually narrowing 

 hollow stalk ; but this stalk must, like the embryonic 

 sac itself, be a double stalk, and consist of a smaller 

 inner stalk within a larger outer one. Fig. 9, E, II. 

 The folds of the splanchnopleure, as they tend to 

 meet and unite in the middle line below, give 

 rise to a continually narrowing hollow stalk of their 

 own, a splanchnic stalk, by means of which the walls of 

 the alimentary canal are continuous with the splanch- 

 nopleuric investment of the yolk-sac, and the interior 

 of that canal is continuous with the cavity inside the 

 yolk-sac. In the same way the folds of the somato- 

 pleure form a similar stalk of their own, a somatic 

 stalk, by means of which the body- walls of the chick 

 are continuous (for some time ; the continuity, as we 

 shall see, being eventually broken by the development 

 of the amnion) with the somatopleuric investment of 

 the yolk-sac ; and the pleuroperitoneal cavity of the 



^ Splanchnon, viscus, pleuron, side. 



