302 UADIATION lUULOGY 



Ilonshaw and Cohen (1940) in that sensitivity decreases as the time after 

 fertilization increases. However, Ilenshaw and Cohen found a small 

 secondary increase in sensitivity in the early prophase stage. Gross 

 (1950) found that Chartoptcrus vg^s exposed to ultraviolet showed some- 

 what the same sensitivity relations as did X-irradiated Arhacia eggs. 

 'Hie eggs were (juite sensitive for the first 30 min, after which the sensi- 

 tivity decreased hut increased again in the period from 40 to 50 min. The 

 simplest explanation of the results of Blum and Price (1950) would be 

 that there is a period after which division is irrevocably determined. 

 However, the results of the other two investigators are not so easily 

 explained, and it seems probable that rather complex factors are involved 

 in the.se changes in sensiti\'ity. 



These reports make it clear that sponluncous recovery from the effects 

 of ratliation occurs in the sea urchin. For ultraviolet, it appears estab- 

 lished that this recovery is a gradual process extending over more than 

 one division. Both Henshaw and Lea treat the X-ray data as thougli 

 recovery were complete by the time division occurs, but Henshaw and 

 his coworkers do not present evidence on later cleavages. In view of this 

 and the statement of Blum . . . Loos (1949), that X- and ultraviolet- 

 irradiated material showed the same behavior, it seems possible that in 

 both cases the recovery process is independent of the occurrence of 

 cleavage. If this is so, as suggested also in a brief statement in Blum 

 et al. (1951), then radiation must affect something which controls the rate 

 at which division occurs instead of simply destroying some material which 

 must be restored to its original amount before any cleavage can take place. 



The separation between cell division and recovery is much more marked 

 in the cihate protozoa. Giese (1939a, 1945b) has shown clearly for 

 Paramecium caudatum that ultraviolet retards several divisions following 

 the irradiation (see Fig. 8-lb). Giese and Reed (1940) have shown the 

 same thing in somewhat less detail for several species of Paramecium. 

 (Jiese (194(ia) has made similar findings for tfie retardation of division by 

 visible light in Paramecium exposed to eosin, and in Blepharisma. Kim- 

 ball et al. (1952) find the following pattern for P. awre/m exposed to mono- 

 chromatic ultraviolet (see Fig. 8-16). The same pattern has been found 

 for wave lengths 2378, 2537, 2()50, and 2804 A. The first division follow- 

 ing irradiation is usually markedly delayed. The next division is also 

 delayed, but less so. Either the third or fourth interval is often extremely 

 long, lasting in some cases for two weeks or more. Finally, recovery of 

 the normal rate is usually complete by the sixth division. It seems possi- 

 ble that at least thn^e prcx'cs.ses should be recognized: (1) retardation of 

 the first division after irradiation, (2) a long but not permanent cessation 

 of division, usually setting in after two or three divisions have occurred, 

 and (3) a relatively small increase in all other division intervals through 

 about the sixth. The relative magnitude of these various effects appears 



