BREEDING HABITS OF THE OPOSSUM HARTMAN. 349 



are a part of the vagina, since the ocstrous changes in the organs are 

 identical. Parenthetically it may be pointed out that, in adaptation 

 to the paired lateral vaginal canals the glans penis of the male is 

 bifurcate. The spermatozoa, to fertilize the eggs, must travel 

 through the canals and the uteri to the Fallopian tubes, where fer- 

 tilization takes place. At birth, however, strangely enough, the 

 fetuses do not follow the lateral vaginal canals to the exterior but 

 break through a "short cut" to the median vagina, as shall appear 

 below in the section dealing with parturition. 



In spite of the paired condition of the uteri the eggs of both sides 

 are simultaneously fertilized — if sperms reach one tube they reach 

 both. Among several hundred pregnant specimens scarcely an ex- 

 ception to this rule has been found. 



THE OPOSSUM AS AN EXPERIMENTAL ANIMAL. 



The possession of two distinct uteri renders the animal ideal for 

 securing accurately timed developmental stages. To do this one 

 uterus is removed under anesthesia and aseptic conditions and the 

 stage of the contained eggs or embryos ascertained. The intact uterus 

 then serves as an ideal incubator for the remaining eggs, which are 

 allowed to develop for a calculated length of time. This technique is 

 one of the factors which have enabled the writer, assisted by Dr. C. 

 H. Heuser formerly of the Wistar Institute, in several weeks of 1917 

 to secure a nearly complete series of stages. The method may, of 

 course, be used with other mammals, e. g., the rabbit, as Bischoff did 

 nearly 100 years ago ; but inflammation is likety to obscure the results 

 where the horns of the uterus are successively ligated. 



To illustrate the application of the method, several sets of eggs are 

 shown in plates 4, 5, and 6. Some of these are discussed below under 

 the heading " Period of gestation." Suffice it at this point to direct 

 attention to two cases. The eggs of animals Nos. 5G0' (fig. 11, pi. 4) 

 and 582' (figs. 24 and 25, pi. 6) had arrived at the identical stage 

 when the animals were killed. Two of these eggs are shown in detail 

 in figure 21, pi. 6. Now No. 582' had developed in 66 hours from 

 0.75 millimeter vesicles (fig. 20, pi. 6) ; No. 560' in 18 hours from a 

 primitive streak stage (fig. 13). It is thus relatively easy to secure 

 almost any desired developmental stage. 



There are other reasons, important to the physiologist, which make 

 the opossum an interesting object for experimentation. For example, 

 because of its' early birth, the study of the opossum must needs con- 

 tribute its share to the final analysis of the cause of birth. In this 

 species, furthermore, there is no structure that may be called a 

 placenta ; hence those who make much of the role of the placenta as 

 an organ of internal secretion will do well to check their results by 



