298 RADIATION HIOLOGY 



that division ck'hiy by nidiatiou ever lasts for more than a few divisions, 

 provided the cells survive at all. The eases in microorganisms in which 

 lasting reduction in rate of multiplication have been found are probably 

 the results of genetic changes quite independeni of the original retarda- 

 tion of division. 



The time course of reccnery may vary greatly in ditferent cases. In 

 some cases, retardation may last for several divisions before complete 

 recovery occurs; in some, recovery may be complete, or nearly so, by the 

 first division, while in others, there seem to be stages during which no 

 recovery occurs. The studies have been concerned mainly with (lualita- 

 tive and ciuantitative descriptions of the time course of recovery with only 

 a small amount of attention being devoted to attempts to influence 

 recovery experimentally. 



The most complete ([uantitative study of division delay has been car- 

 ried out with echinoderm eggs and sperm, chiefly those of Arbacia. 

 Most of the investigations with ionizing radiations have been concerned 

 with the first cleavage only. However, Miwa et al. (1939a) irradiated 

 unfertilized eggs of Pseudocentrotus depressus with /3 particles from radon 

 and recorded the time to both the first and the second cleavage. The 

 data suggest to the reviewer that the interval between the first and second 

 cleavages may be slightly longer than normal at higher doses although the 

 the authors say that ''there is little or no delay in the . . . second divi- 

 sion" (see Fig. 8-la). Yamashita et al. (1939) exposed fertilized eggs of 

 this same sea urchin to X, y, and rays and stated that they could find 

 no evidence that irradiation during most of the period before the first 

 division had any marked effect on later cleavages. However, Blum 

 . . . Loos (1949) mention in an abstract that they have obtained the 

 same results with X rays and with ultraviolet radiation in fertilized 

 Arbacia eggs. Presumably, this includes delay in cleavages later than 

 the first. The method used by Blum and his coworkers (see Blum and 

 Price, 1950) is probably better designed to detect small difi'erences in 

 clea^'age times than were those of previous investigators. It therefore 

 seems probable that the effects of ionizing radiations last for more than 

 one cleavage but are, in most experiments, of small importance in intervals 

 beyond the first interval following the treatment. 



The recovery of sea urchin eggs from cleavage delay by ionizing radia- 

 tions has been investigated by two groups of workers, a Japanese group 

 (Miwa, Mori, and Yamashita) and Ilenshaw and his collaborators. 

 Henshaw's results have been interpreted theoretically by Lea (1938a, b, 

 1947). 



Ilenshaw (1932, 194()c) and Miwa d al. (1939a) found that the longer 

 the period between irradiation of eggs and insemination with unirradiated 

 sperm, the less the effect. In other words, recovery occuncd between 

 irradiation and insemination. Irradiation of sperm also brings about 



