ALIMENT A R Y SYSTEM SUMMAR Y. 



48: 



justified by the increase of absorptive and digestive surface. There 

 are regular longitudinal folds in My. \ine, 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 maybe 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 step be affected as usual by natural selection. 



ALIMENTARY SYSTEM. SUMMARY. 



REGION OF THE GUT. 



OUTGROWTHS. 



ASSOCIATED STRUCTURES. 



Mouth cavity. 



or Stomoda:um, 



or Fore-gut, 

 originating as an ectodermic 

 invagination. 



Oral part 

 hypophysis. 



of the 



Pharynx, gullet or oeso- 

 phagus, stomach, small in- 

 testine, large intestine, and 

 rectum ; = the mesenteron or 

 mid-gut, originating from 

 the cavity of the gastrula. 

 the archenteron or primitive 

 gut ; lined by endoderm. 



Anal region, 



or Proctodaeum, 



or Hind-gut, 



originating as an ectodermic 

 invagination. 



Thyroid* and the 



Thymusf gill-clefts. 



Air bladder ; lungs. 



Liver. 



Pancreas. 



Allantois. 



The pancreas is 

 usually the result of 

 two ventral out- 

 growths and a dorsal 

 one. In Cyclostomes 

 and Elasmobranchs it 

 seems to have but 

 one rudiment ; in the 

 sturgeon four. 



Teeth. 



Salivary glands, 

 Tongue. 



With the several out- 

 growths the surrounding me- 

 soderm becomes associated, 

 often to a great extent. 



Note also the origin of 

 the notochord as an axial 

 differentiation of cells along 

 the mid-dorsal line of the 

 embryonic gut. 



In some Fishes, all Amphi- 

 bians, all Sauropsida, and 

 the Prototherian Mammals, 

 the terminal part of the 

 gut is a cloaca or common 

 chamber, into which the 

 rectum, the urinary, and the 

 genital ducts open. 



Body cavity. In Amphioxus the coelom arises as pouches 



