502 STRUCTURE OF VERTEBRATA. 



seems not unlikely that the alimentary tract was originally a 

 ciliated tube. 



At the posterior end an ectodermic invagination or proctodoeum 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, caeca, and coils, which would be 

 justified by the increase of absorptive and digestive surface. There 

 are regular longitudinal folds in Myxine, cross-folds traversing these 

 would form crypts, which may be exaggerated into the pyloric caeca of 

 Teleosteans and Ganoids, while other modifications would give rise to 

 "spiral valves" and the like. In the same way it may be suggested 

 that the numerous important outgrowths of the mid-gut, such as lungs, 

 liver, pancreas, and allantois, so thoroughly justified by their usefulness, 

 may at first have been due to necessary conditions of growth to the 

 high nutrition, rapid growth, and rapid multiplication of the endoderm. 

 It may be noted that in the development of the Amphibian Necturus, 

 there are hints of more numerous endodermic diverticula (Platt). It is 

 also said that the hypochorda a transitory structure arising below and 

 subsequent to the notochord, is in part due to a series of dorsal out- 

 growths from the gut (Stohr). Even the notochord, which arises as 

 a median dorsal fold, may be speculatively compared to a typhlosole 

 folded outwards instead of inwards. The future elaboration of the 

 organs which arise as outgrowths of the gut would, however, 

 depend on many factors, such as their correlation with other parts 

 of the body, and would at each st^p be affected as usual by natural 

 selection. 



[TABLE. 



