418 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



certain organs connected with the alimentary canal 

 such as the mesentery, stomodaeum, etc., which are not 

 hypoblastic in origin. 



The origin of the hypoblast, and the process of 

 folding by which the cavity of the mesenteron is 

 established have already been described. The mesen- 

 teron may be considered under three heads. 



1. The anterior or respiratory division of the 

 mesenteron. The pharynx, thyroid body, Eustachian 

 tube, tympanic cavity, oesophagus, trachea, bronchi, lungs 

 and stomach are developed from this portion, and their 

 development in the Mammal so closely resembles that in 

 the Chick that it is unnecessary for us to add to the 

 account we have already given in the earlier part of this 

 work. 



This section of the alimentary canal, as in the Chick, 

 is distinguished in the embryo by the fact that its walls 

 send out a series of paired diverticula which meet the 

 skin, and, after perforation has been effected at the 

 regions of contact, form the visceral clefts. 



2. The middle division of the mesenteron, from 

 which the liver and pancreas are developed, as in the 

 Chick, forms the intestinal and cloacal region and is at 

 first a straight tube. It remains for some time connected 

 with the yolk sack. 



The Cloaca appears as a dilatation of the mesen- 

 teron which receives, as in Aves, the opening of the 

 allantois almost as soon as the posterior section of 

 the alimentary tract is established. The eventual 

 changes which it undergoes have already been dealt 

 with in connection with the urinogenital organs. 



The intestine. The posterior part of this becomes 



