830 RADIATION BIOLOGY 



first reappearance of mitotic spermatogonia to the production of sperm, is 

 approximately four weeks in the mouse. The interesting results of 

 Howard and Pelc (1950), who used autoradiographs, following P 32 injec- 

 tion, to measure the times taken by various stages of spermatogenesis in 

 the mouse, show that the interval from spermatogonial metaphase to 

 immature sperm is more than 10 days. 



The surviving cells have been referred to as spermatogonia. This is the 

 view taken by most authors, including Hertwig (1938b) who identifies the 

 cells as spermatogonia with "dusty," pale, oval nuclei. Other interpreta- 

 tions are mentioned by Hertwig. Among the spermatogonia there is 

 great variety in size and appearance of nuclei. The frequencies of these 

 types at various intervals after irradiation, together with interpretations 

 of what stages in spermatogonial divisions the types represent, are also 

 presented by Hertwig. 



From the histological observations it appears that matings made in the 

 initial period of fertility following irradiation utilize sperm that were 

 mature at the time of irradiation, and that copulations occurring in the 

 latter part of the period may use some sperm that matured from cells that 

 were spermatids or spermatocytes when irradiated. The period of 

 temporary sterility corresponds to the interval in which replacement of 

 later germ cell stages has not yet been completed owing to the depletion 

 of spermatogonia. Matings made after the sterile period use sperm that 

 were in the spermatogonial stage at the time of irradiation. The sterile 

 period is thus a useful marker for separating genetic changes induced in 

 spermatogonia from those induced in later germ cell stages. The reduc- 

 tion in litter size in the initial fertile period is not accounted for by the 

 histological findings described above, but is explained by the results 

 described in the next section. 



The breeding results and histological changes in the gonads following 

 exposure to neutrons or y rays, and following the use of X rays on 

 females, will be described only briefly since these treatments have had 

 only limited use in genetic studies. 



Snell and Aebersold (1937) have shown that the effects on reproduc- 

 tion, following a single exposure of male mice to cyclotron neutrons, are 

 similar to those produced by X rays. However, measured on the 

 roentgen scale of a Victoreen condenser-type dosimeter, neutrons are 

 from five to six times as potent as X rays in reducing the litter size in pre- 

 sterile-period matings. 



The effects on the gonads and on fertility of long exposure to low 

 intensity 7 radiation have been described by Lorenz et al. (1947) and 

 Eschenbrenner et al. (1948). Vigorous hybrid male mice exposed to a 

 total dose of 1100 r at 8.8 r per day from a radium source and then 

 removed from the radiation field gave litters of reduced size in early 

 matings and normal litters later. Those exposed to 1760 r at the same 



