INTESTINAL NUTRITION IN THE OPOSSUM 351 



combined width of the external layers is relatively greater than 

 it is in the preceding portions of the digestive tract. The epi- 

 thelial lining of the colon consists of undifferentiated low col- 

 umnar cells with large elongated nuclei and homogeneous granular 

 cytoplasm. There are many thin-walled blood-vessels in the 

 loose mesenchyme of the submucosa. The muscular coat is a 

 thin layer made up of about three rows of myoblasts without 

 differentiation into longitudinal and circular coats. The layer 

 of mesenchyme surrounding the muscularis is slightly denser 

 than that constituting the submucosa. 



In the pouch-young : The large intestine grows a little more 

 rapidly than the other portions of the intestine, but in the pouch- 

 young studied it is still the least differentiated region of the 

 intestinal tube. The girth is only a little greater than that of the 

 smallest of the coils of the ileum. The caecum remains a long 

 slender lateral outgrowth as in the younger stage. There is in 

 addition a short epithelial diverticulum which has grown out 

 from the large intestine near its union with the small intestine. 

 The significance of this diverticulum is not known. 



The mucosa has grown so that it is about twice its former 

 width. The tall columnar cells extend from the cuticular border 

 to a distinct basement membrane. The cytoplasm is evenly 

 stained and undifferentiated. The nuclei are confined largely to 

 the lower portion of the cells, but they are not arranged in a 

 single row; with the folding of the epithelial tube the regular 

 distribution of the nuclei is apparently disturbed ; they are found 

 in groups opposite the folds and arranged in a triple layer. The 

 lumen is pentagonal in transverse section. The increased caliber 

 of the large intestine may be accounted for by the increase in 

 size of the lumen and to the growth in thickness of the mucosa; 

 the external layers, however, are relatively thinner than in the 

 embryo. 



DISCUSSION AND SUMMARY 



The period of gestation in the opossum is very short; the 

 interval from copulation to parturition is only of thirteen days' 

 duration, and of this period three days have passed before the 



