CHAPTER X. 

 The Alimentary Canal. 



The present Chapter completes the history of the primitive 

 alimentary canal, whose formation has already been described. 

 In order to economise space, no attempt has been made to give 

 a full account of the alimentary canal and its appendages, but 

 only those points have been dealt with which present any 

 features of special interest. 



The development of the following organs is described in 

 order. 



(1) The solid oesophagus. 



(2) The postanal section of the alimentary tract. 



(3) The cloaca and anus. 



(4) The thyroid body. 



(5) The pancreas. 



(6) The liver. 



(7) The subnotochordal rod. 



The solid oesophagus, 



A curious point which has turned up in the course of my 

 investigations is the fact that for a considerable period of 

 embiyonic life a part of the oesophagus remains quite solid and 

 without a lumen. The part of the oesophagus to undergo this 

 peculiar change is that which overlies the heart, and extends 

 from the front end of the stomach to the branchial region. At 

 first, this part of the oesophagus has the form of a tube with 

 a well-developed lumen like the remainder of the alimentary 

 tract, but at a stage slightly younger than K its lumen becomes 

 smaller, and finally vanishes, and the original tube is replaced 

 by a solid rod of uniform and somewhat polygonal cells. A 

 section of it in this condition is represented in PL x. fig. Sa. 



At a slightly later stage its outermost cells become more 

 B. 15 



