GENETIC EFFECTS IN MAMMALS 



829 



and interstitial tissue, are resistant to radiation, and that spermatogonia 

 are quite sensitive. 2 Reduction in numbers is found first in the spermato- 

 gonia. The later germ cell stages disappear in the order in which they 

 are formed in spermatogenesis and at approximately the same rate as 

 that of the spermatogonial disappearance (Hertwig, 1938b; Eschen- 

 brenner and Miller, 1950; Fogg and Cowing, 1951a, b). As there is 

 evidence that the spermatocytes and spermatids present at the time of 

 irradiation complete their development (Schaefer, 1939; Eschenbrenner 

 and Miller, 1950), the disappearance of these classes is attributable to 

 failure of replacement by the depleted spermatogonia. Since Fogg and 

 CoAving (1951a, b, 1952) nowhere mention this commonly accepted view, 

 and even wrongly imply that Eschenbrenner and Miller adopted another 

 explanation, it must be concluded that their own suggestion, that the 

 progressive disappearance of later stages reflects a delay in response to 

 irradiation correlated with cell specialization, was made in unawareness 

 of the consequence of loss of spermatogonia. The number of spermato- 

 gonia rapidly reaches a minimum. The reduction in number was 

 measured, in mice, by Hertwig (1938b) for a dose of 800 r, by Eschen- 

 brenner and Miller (1950) for 400 r and, less quantitatively, by Fogg and 

 Cowing (1951a, b) for other doses (Table 12-3). The number of sperma- 



Table 12-3. Reduction in Number of Spermatogonia in Mice Exposed to 



Various Doses of X Rays 



° No observation made at earlier time. 



6 Fogg and Cowing state that normal testis shows spermatogonia in 80 per cent of cross sections of 

 tubules. 



c Based on reduction of resting spermatogonia to 3.3 per cent and of mitotic spermatogonia to 

 per cent. Eschenbrenner, Miller, and Lorenz (1948) give the ratio of these two types in testes of unirra- 

 diated mice of similar age as 8.9: 11.1. 



d Hertwig found a total of 9 spermatogonia in the 59 cross sections examined and states that a cross 

 section of a tubule of a normal testis contains 20 spermatogonia. 



togonia then increases, and eventually the later germ cell stages reappear 

 in the order in which they disappeared (Hertwig, 1938b; Eschenbrenner 

 and Miller, 1950; Fogg and Cowing, 1951a, b). The time taken, from the 



2 Unpublished data, obtained by E. F. Oakberg at this laboratory since this manu- 

 script was submitted, show that primary spermatocytes, as well as spermatogonia, 

 show appreciable sensitivity to radiation. 



