204 Vertebrate Embryology 



Up to this time the alimentary canal lies 

 very high up in the body-cavity, being sepa- 

 rated from the notochord and the aortae by 

 only a broad, thin layer of mesoblast (Figs. 48, 

 Ph and 70). During this day, however, the 

 digestive canal, for a greater part of its 

 length, draws away from the upper side of 

 the body-cavity, to which it remains attached 

 by a constantly narrowing band of tissue, the 

 mesentery (Figs. 71 and 73). 



The mesentery is composed of mesoblast 

 that is continuous with that which surrounds 

 the entoblast of the digestive canal, and this 

 mesoblast consists of an undifferentiated 

 middle layer, in which the blood vessels are 

 later developed, and a superficial layer of 

 epithelium, continuous with the epithelial 

 lining of the peritoneal cavity. In the an- 

 terior part of the fore-gut the withdrawal of 

 the digestive tract from the notochord is very 

 slight, as there is little or no development 

 of the mesentery in the region of the 

 oesophagus. 



The anterior end of the cesophageal region 

 is broadened out to form the pharynx, where 

 the gill clefts are developed, as has been de- 

 scribed. The hinder end of the cesophageal 



