VI.] THE (ESOPHAGUS. 173 



tic tissue which reaches from the neighbourhood of the 

 notochord, and becomes continuous with the mesoblas- 

 tic coating which wraps round the hypoblast of the 

 canal. This flattened band is the mesentery, shewn 

 commencing in Fig. 65, and much more advanced in 

 Fig. 68, M. It is covered on either side by a layer of 

 flat cells forming the epithelioid lining of the peritoneal 

 membrane, while its interior is composed of indifferent 

 tissue. 



The front division of the digestive tract consists of 

 three parts. The most anterior part, the oesophagus, 

 still ending blindly in front reaches back as far as the 

 level of the hind end of the heart ; and is divided into 

 two regions, viz. an anterior region, characterized by the 

 presence of the visceral clefts, whose development has 

 already been dealt with, and a posterior region without 

 such clefts. 



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. 60, 

 Ig), of whose formation we shall speak directly, take 

 their origin. 



The portion of the digestive canal which succeeds 

 the oesophagus, becomes towards the close of the third 

 day somewhat dilated (Fig. 60, 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 com- 

 plete closing in of the alimentary canal ceases, and where 

 the splanchnopleure-folds spread out over the yolk, 

 This short tract is nevertheless clearly marked out as 



