REPRODUCTION 65 



Butcher (26) has described regeneration of rat ovaries following ligation. 

 At eight or ten days the ligatcd ovaries consisted almost entirely of degener- 

 ate tissue. In the periphery of the ovary, particularly in the region where a 

 cavity had persisted between the gonad and capsule, small or primary 

 follicles were found, but in no case did the number encountered in one ovary 

 exceed twenty. Recovery was rapid, the ovary being practically normal at 

 thirty to thirty-four days. These cases of regeneration in the adult rat and 

 mouse are excellent evidence that ovogenesis can occur in sexually mature 

 animals. 



In view of the extent of ovarian regeneration following partial removal it 

 is noteworthy that the ovaries of mice sterilized with x-rays, either at birth 

 or later in life, never regenerate any germ cells although they remain 

 functional in regulating the estrous cycle (22, 98). 



Robertson (in) has described the successful transplantation of ovaries 

 between mice of the same inbred strain. 



The estrous cycle. — Our present knowledge of the estrous cycle in rodents 

 dates from the discovery of Stockard and Papanicolaou that the cellular 

 contents of the vagina undergo cyclical changes and that by observing these 

 changes in vaginal smears the successive stages of the estrous cycle can be 

 accurately followed and the time of heat determined. The estrous cycle of 

 the mouse has been studied by /Allen 1(3), Rietschel (no), Clauberg (29), and 

 others. Long and Evans (83) have pubHshed a very thorough study of the 

 cycle in the rat. The following description is based on Allen's studies except 

 as otherwise noted. 



Divisions of the estrous cycle. — ^The estrous cycle of the mouse and rat 

 is conveniently divided into 5 stages, namely, 'proestrus, estrus or heat, 

 metestrus-i, metestrus-2, diestrus. The first two are anabolic stages during 

 which active growth is in progress in various parts of the genital tract. 

 They culminate in ovulation and, where mating occurs, in fertilization. The 

 second two, metestrus-i and metestrus-2, are catabolic stages characterized 

 by degenerative changes in the genital tract. The last, diestrus, is a period 

 of quiescence or slow growth. The characteristics of each stage are sum- 

 marized in Table 4. 



External signs of estrus. — There is a tendency at proestrus and estrus 



for the vulva to show swelling and congestion, and for the vaginal orifice to 



_gap£, but these appearances are so variable as to be unreliable signs of heat. 



The onset of heat in the rat can be accurately determined by the '' copulaton,- 



response" (65, 137). 



