REPRODUCTION 59 



probably does not always occur at this time. Data on the occurrence of the 

 first estrus and the first mating have been pubHshed by Mirskaia and Crew 

 (95, 96) and are summarized in Table 3. 



As this table shows, the time of the first fertile mating varies greatly. 

 Commonly it occurs_ at seven to ten weeks. Thirty-nine days is exception- 

 ally early. Maturity in males occurs at about the same time as in females, 

 or perhaps somewhat later./ 



The useful breeding period of most inbred females terminates when they 

 reach ten or twelve months of age, for though litters may continue to be 

 produced after this, breeding is apt to be irregular and the litters small. 

 Hybrid females usually give fair sized litters and breed regularly until 

 sixteen or eighteen months of age. Males will usually breed several months 

 longer than females of the same stocky 



Occasionally mice live to^hree years of age or even a few months past 

 this. ^^ 



Ovogenesis. — The problem of the origin of the female germ cells has been 

 the subject of extensive study. The following description is based on the 

 most important recent papers deaHng with ovogenesis in mice and rats and 

 does not present all the conflicting viewpoints found in much of the older and 

 some of the more recent literature. All statements are based on work with 

 the mouse unless otherwise specified. Investigations in this field have been 

 ably reviewed by Heys (67) and Pincus (108). 



Beginning at about nine (18) to eleven (72) days post coitus, the gonads 

 of mouse embryos contain so-called primordial or primitive germ cells, char- 

 acterized by their large size and by the clear appearance of the cytoplasm. 

 These are present in both male and female gonads which at this early stage 

 are indistinguishable. At this same time or slightly later, cells closely 

 similar in appearance may be seen in tissues adjoining the gonads (18), a fact 

 that has led to extensive speculation as to their place of origin and possible 

 migrations. The view, at one time commonly held, that they migrate into 

 the ovary and there give rise to the germ cells is not supported by recent 

 evidence. The young primordial ova show numerous mitoses, though these 

 soon cease. Proliferation of ova from the germinal epithelium, however, 

 continues. By the twelfth to fourteenth day post coitus the nuclei of the 

 oldest ova enter on the complex series of stages characteristic of the meiotic 

 prophase, all of them reaching at least the pachytene stage by the time of 

 birth (18, 25). SHghtly before birth some of the primordial ova have begun 

 to be surrounded by follicle cells, and by three (55, 70) to six (18) days post 

 partum all the oocytes in the ovary proper have a follicular epithelium. The 



