INTESTINAL NUTRITION IN THE OPOSSUM, 355 



In many places between the villi there are patches of epithelial 

 cells which are smaller and more darkly stained than those 

 covering the villi. In the distal ends of the cells covering the 

 villi one sees very prominent masses which can usually be re- 

 solved into granules with the high power. These granules stain 

 brilliantly with eosin and with acid fuchsin. The chief interest 

 in them lies in speculation as to whether they are products of 

 secretion or of absorption; no data, other than that afforded by 

 histological examination, have been obtained. 



The large intestine had changed to a much smaller degree 

 relatively in the transformation from the late embryo to the 

 pouch-young. Although the external caliber of the colon has 

 increased considerably in the interval of growth, there is but 

 little advance in the epithelial or mesoblastic differentiation. In 

 the pouch-young the epithelial lining-cells have doubled in 

 height; the lumen is considerably increased in size and the mes- 

 enchyme is markedly condensed. These morphological changes 

 are, however, much less pronounced than are the differen- 

 tiations which have occurred in the upper portions of the small 

 intestine. The process of differentiation apparently proceeds 

 from above downward, in the direction of the physiological 

 passage of food-stuffs. The relatively slight dilation of the colon 

 and the histological findings indicate that the residue of the food- 

 stuffs reaching the colon in the opossum pouch-young is relatively 

 small; practically all of the milk must be disposed of in the upper 

 portions of the alimentary canal. 



In summary it may be said that the changes in the gastro- 

 intestinal canal from the late embryo to the early pouch-young, 

 in which the digestion of milk must occur, are marked. Certain 

 of the changes, as the enormous dilatation of the stomach, must 

 be largely due to mechanical causes; other alterations, as in the 

 formation of villi and the differentiation of the epithelial cells 

 in the upper parts of the small intestine, must be accounted for 

 by the natural processes of development of the embryo, aided 

 or hastened possibly by the early functional requirements of the 

 organs. 



