558 



STRUCTURE OF VERTEBRATA 



At the posterior end an ectodermic invagination or proctodaeum 

 meets the closed archenteron, and at the junction the two epithelial 

 layers give way, so that an open tube is formed. 



The formation of the anus does not take place close to the posterior 

 end of the primitive gut, but at a point some short distance in front of 

 this. In consequence the so-called post-anal gut is formed. This is 

 continuous with the neurenteric canal, and so communicates with the 

 neural canal. The post-anal gut attains in Elasmobranchs a relatively 

 considerable length. It has been very frequently found in Vertebrates, 

 and is probably of universal occurrence. After a longer or shorter 

 period it becomes completely atrophied, and with it the communication 

 between neural and alimentary canals is completely destroyed. In 

 some Fishes and Amphibians the anus is formed directly from the 

 blastopore. 



Speculative. — The primitive gut was probably a smooth straight 

 tube, but the rapid multiplication of well-nourished cells would tend 

 to its increase in diameter and in length. But on increase in both 

 directions the slower growth of the general body would impose limita- 

 tions, and in this we may find the immediate growth-condition deter- 

 mining the origin of folds, crypts, cseca, and coils, which would be 

 justified by the increase of absorptive and digestive surface. There 



Alimentary System. — Summary 



