TYPES OF (ESTROUS CYCLE 



29 



between the development of pregnancy and of pseudo- 

 pregnancy. 



The Opossum. The opossum was originally thought to be 

 monoestrous, like Z)(^s\7/r//s, but Hartman's (271, 278] detailed 

 study of the cycle shows it to be polyoestrous. The di- 

 oestrous cycle in the opossum is, however, complicated by a 



CEstrus Ovulation 



— I — I — r~ 

 Graafian 

 follicle 



Parturition 



CEstrus 

 + 







L— J \ \ 1 1 1 1 1 I I L_L_1 I I I I \ I I I I I I I 



10 



15 



20 



25 days 



Fig. 14. — Diagram of CEstrous Cycle in the Opossum. 

 (From Hartman). 



definite period of pseudo-pregnancy which, as a rule, is not 

 well marked in polyoestrous animals. The recent work on the 

 opossum has been greatly facilitated by the fact that this Mar- 

 supial, like the rat, mouse, and guinea-pig, shows characteristic 

 oestrous changes in the vagina. With the aid of a vaginal smear 

 technique, Hartman has been able to study in detail the various 

 phases of the cycle, the total length of which is about twenty- 

 eight days. He points out, however, that its duration may be 



