CHAPTER XXV. 



THE ALIMENTARY CANAL AND ITS APPENDAGES, 



IN THE CHORDATA. 



THE alimentary canal in the Chord ata is always formed of three 

 sections, analogous to those so universally present in the Inverte- 

 brata. These sections are (1) the mesenteron lined by hypoblast; 

 (2) the stomoditum or mouth lined by epiblast, and (3) the procto- 

 diimm or anal section lined like the stomodseum by epiblast. 



Mesenteron. 



The early development of the epithelial wall of the mesenteron 

 has already been described (Chapter XL). It forms at first a simple 

 hypoblastic tube extending from near the front end of the body, 

 where it terminates blindly, to the hinder extremity where it is 

 united with the neural tube by the neurenteric canal (fig. 420, ne). 

 It often remains for a long time widely open in the middle towards 

 the yolk-sack. 



It has already been shewn that from the dorsal wall of the 

 mesenteron the notochord is separated off nearly at the same time 

 as the lateral plates of mesoblast (pp. 243 249). 



The subnotochordal rod. At a period slightly subsequent to the 

 formation of the notochord, and before any important differentiations 

 in the mesenteron have become apparent, a remarkable rod-like 

 body, which was first discovered by Gotte, becomes split off from the 

 dorsal wall of the alimentary tract in all the Ichthyopsida. This 

 body, which has a purely provisional existence, is known as the sub- 

 notochordal rod. 



It develops in Elasmobranch embryos in two sections, one situated in 

 the head, and the other in the trunk. 



The section in the trunk is the first to appear. The wall of the 

 alimentary canal becomes thickened along the median dorsal line (fig. 

 412, x), or else produced into a ridge into which there penetrates a narrow 

 prolongation of the lumen of the alimentary canal. In either case the 

 cells at the extreme summit become gradually constricted off as a rod, 



