THE EMBRYOLOGY OF THE OPOSSUM 13 



bones. Under natural conditions they eat, among other 

 things, small vertebrates (frogs, lizards, salamanders, small 

 birds, etc.) entire bones and all. In our colony they were 

 getting plenty of meat (beef, beef heart, and liver) but no 

 bones. So after that we sprinkled bone meal into the food 

 pans every day, and that cured the rickets. This suggests a 

 curious calcium and phosphorus metabolism which might be 

 worthy of further study. As their parathyroid glands are 

 also peculiar, there may be an interesting correlation here. 



After the addition of bone meal to the diet the animals grew 

 to full, normal adult stature and proportions, and did not 

 die prematurely; but still they did not breed. Examination 

 of the testes of the males raised in the colony showed plenty 

 of normal-looking, mature spermatozoa; but the ovaries of 

 the females were definitely atrophic, and contained few or no 

 mature graafian follicles. Hartman ('23) had reported some 

 evidence that this condition of the ovary might be associated 

 with inadequate opportunity for exercise, so we tried enlarg- 

 ing the pens and introducing devices for climbing. The nest 

 boxes in which the animals sleep during the day were hung on 

 racks on the walls of the colony rooms, and the food pans 

 were placed on the floor. The animals had to climb down to 

 feed during the night, and with the return of daylight, if 

 they wanted a dark hole in which to hide, they had to climb 

 back up. This they invariably did, rather than sleep on the 

 floor in open daylight, so a certain amount of exercise was 

 forced upon them, but they took advantage of their oppor- 

 tunities to a greater extent than that, and at night I often 

 found them chasing each other up and down the tree branches 

 which led to the nest boxes. 



After that change the females began to ovulate, and we got 

 our first litters from animals which had been raised in cap- 

 tivity. In the spring and summer of 1935 we had 250 young 

 born in the colony. This year (1937) we have some third- 

 generation, captive-born young; so we feel that at least our 

 major problems must have been solved. 



