ALIMENTARY SYSTEM — SUMMARY. 



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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, cfeca, 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 (Piatt). 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 outgiowths 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. 



Alimentary System. — Summary. 



