128 



THE THIRD DAT. 



[chap 



kind end of the heart. Its transverse section, which up to 

 the end of the second day was somewhat crescent-shaped, with 

 the convexity downwards, becomes on this day more nearly 

 circular. Close to its hinder limit, the lungs (Fig. 42, Ig), of 

 whose formation we shall speak directly, take their origin. 



Fig. 42. 



Diagram of a portion of the Digestive Tract of a Chick upon the 

 Fourth Day. (Copied from Gotte.) 



The black inner line represents the hypoblast, the outer shading the mesoblast. 

 1(1. lung-diverticulum with expanded termination, forming the primary 

 lung- vesicle. St. stomach. I. two hepatic diverticula with their terminations 

 united by solid rows of hypoblast cells, p. diverticulum of the pancreas 

 Avith the vesicular diverticula coming from it. 



The portion of the digestive canal which succeeds the 

 CBSophagus, becomes towards the close of the third day 

 somewhat dilated (Fig. 42, St) ; the region of the stomach is 

 thus indicated. 



The hinder or pyloric end of the stomach is separated by 

 a very small interval from the point where the complete 

 closing in of the alimentary canal ceases, and where the 

 splanchnopleu re-folds spread out over the yolk. This short 

 tract is nevertheless clearly marked out as the duodenum by 

 the fact that from it, as we shall presently point out, the 

 rudiments of ducts of the liver and pancreas are beginning to 

 be formed. It forms the third part of the front division. • 



The posterior division, corresponding to the great intestine 

 and cloaca, is from its very first formation nearly circular in 



