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STRUCTURE OF VERTEBRATA. 



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 (Piatt). 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 step be affected as usual by natural selection. 



It is often said that, in some cases at least, as in lamprey, frog, and 

 newt, the blastopore or opening of the primitive gastrula cavity persists 

 as the anus of the adult, but it seems doubtful whether the anus is 

 not always a new formation. In many cases, at least, an ectodermic 

 invagination or proctodfeum meets the closed archenteron, and at the junc- 

 tion 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 re- 

 latively 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 com- 

 munication between neural and alimentary canals is completely destroyed. 



Alimentary System. — Summary. 



