TYPES OF (ESTROUS CYCLE 



45 



The period of oestrus is marked by very definite clianges in 

 tlie vagina and uterus, and by follicular maturation in the 

 ovaries. Changes in the mammary gland may also occur during 

 the ordinary unmated cycle. 



Ovarian cycle. The morphological picture of the ovarian cycle 

 is complicated by the fact that the corpora lutea survive 

 histologically long after their function is in abeyance, and in the 

 non-mated adult animal four or five sets of corpora lutea. 



n.c.l. 



Fig. 19. — Ovary of Mouse (Pasini Stain), showing two 

 SETS OF Corpora Lutea. 



n.c.l. new corpora lutea; o.c.l. old corpora lutea. 



representing previous ovulations, may be found at one time. 

 A large number of medium-sized follicles are present in the 

 ovary of the mouse at the beginning of dioestrus, and in this 

 large group it is difficult or impossible to pick out the set destined 

 to ovulate at the next oestrous period. At about the middle of 

 dioestrus, however, certain of the follicles undergo a rapid phase 

 of growth. This growth phase becomes more rapid during 

 prooestrus, and culminates in ovulation during the period of 

 oestrus — late in oestrus according to Allen (6), but earlier 

 according to other workers (82). Long and Evans state that in 

 the rat ovulation may occur any time after eighteen hours from 



