CHAPTER XXV. 



THE ALIMENTARY CANAL AND ITS APPENDAGES, IN 



-THE CHORDATA. 



THE alimentary canal in the Chordata is always formed of 

 three sections, analogous to those so universally present in the 

 Invertebrata. These sections are (i) the mesenteron lined by 

 hypoblast ; (2) the stomodaeum or mouth lined by epiblast, and 

 (3) the proctodaeum or anal section lined like the stomodaeum by 

 epiblast. 



Mesenteron. 



The early development of the epithelial wall of the mesenteron 

 has already been described (Chapter XI.). 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, 7ie). 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. 292 300). 



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 subnotochordal rod. 



