DEVELOPMENT OF THE OPOSSUM 13 



diameter are harder to see than the much smaller unsegmented 

 eggs, for the former are perfectly hyaline and the latter possess 

 a white core, the egg proper, surrounded by the transparent shell 

 and albumen layer. After picking out all of the eggs which can 

 be found by a search among the folds of the mucosa, the uterus 

 is turned inside out and shaken in the dish of Ringer's solution. 

 In this way usually several eggs are recovered which would 

 otherwise be overlooked. A few cubic centimeters of fixing solu- 

 tion, such as Bouin's, added to the solution will facilitate the 

 finding of eggs in the bottom of the dish. They can be located 

 by means of the peculiar way in which they roll when agitated 

 by a gentle stream from a pipette, for the eggs differ in this 

 respect from tissue fragments which collect at the bottom of 

 the dish. 



Eggs were forced out of the Fallopian tubes by means of a 

 stream of Ringer's solution sent through the tube with a fine- 

 pointed glass pipette. 



Aseptic operation was resorted to in about twenty cases in 

 the hopes of securing two stages from the same animal. As 

 opossums have a uterus duplex, they are well adapted for such 

 experiments, for the removal of the uterus and the ovary of one 

 side is a comparatively simple matter. The plan has the obvious 

 advantage of enabling one to note the time of development with 

 accuracy. If unfertilized eggs are found, moreover, the animal 

 may be allowed to come into heat a second time, which happens 

 about one month after the first. Recovery is prompt, and the 

 wound usually heals perfectly. 



Unfortunately, in these experiments unfertilized eggs were 

 found in most cases at the time of the first operation. Thus 

 from female No. 109, the left uterus was removed 6 days and 4 

 hours after copulation and 23 eggs recovered. Six hours later 

 the other uterus was removed and 22 eggs recovered. Both 

 lots, however, proved to be unfertilized and the eggs had begun 

 to break up. Greater fortune, however, attended the study of 

 female No. 114. This animal was operated on 5 days and 4 hours 

 after copulation. The eggs had just been shed, as indicated by 

 fresh stigmata on the ovary, but the eggs were not found, prob- 



