THE EMBRYOLOGY OF THE OPOSSUM 173 



more than pregnancy will. The changes in the mammary 

 glands and genital organs during pseudopregnancy are so 

 similar to those of true pregnancy that so far no one has been 

 able to distinguish the two by any means but one removing 

 one whole uterus and opening it to see whether it contains 

 embryos or not. Selenka, Hartman and I have all used this 

 method. If the technique has been sterile and hemorrhage 

 has been avoided, the incision may then be sutured and the 



Fig. 55 Photograph of stage 35 (17175). 



remaining uterus left for any desired number of hours or days 

 before being removed in turn. Or it may be allowed to com- 

 plete its normal gestation. As the embryos in the two uteri 

 are always at the same stage of development, by removing 

 the two at different times one can obtain accurate information 

 about the intervals between the different stages of develop- 

 ment. The operation does not retard the development of the 

 embryos in the untouched uterus or interfere with parturi- 

 tion in any way. 



