PROTOZOA AND INVERTEBRATE EGGS 299 



cleavage delay, l)ut there is no recovery if sperm are kept for a time l)efore 

 they are used for insemination (Henshaw, 1940a; Miwa et al., 1939a; 

 Mori ct al., 1989). The cleavage delay produced by irradiated sperm can 

 be shown to increase as a linear function of the dose. According to Lea 

 (1947), the data of Henshaw (1940a) show an increase of 25 min in 

 cleavage delay for each doubling of the dose. 



Lea (1938a, b) shows that the recovery in eggs can be adequately repre- 

 sented by an exponential decay of the original effect with time according 

 to the expression e~'^^, where / is the time after irradiation and T is a con- 

 stant. The value of T was calculated to be 35 min for the fertilized egg 

 and 104 min for the unfertilized egg. 



For Arbacia, Henshaw (1940b) has shown that there is little if any 

 delay in stages before the prophase of the first cleavage. Most of the 

 delay is in the prophase, with minor delays in the later stages of mitosis. 

 Yamashita ct al. (1940) find major delays in the late nuclear fusion and 

 prophase stages for Pseudocentrotus and Str'ongylocentrotus. Henshaw 

 and Cohen (1940) irradiated eggs at different times after fertilization and 

 found that the effect produced by a given dose increased for the first 

 10-15 min and then declined, so that by the end of prophase there was 

 little if any effect on the time of the first cleavage. The decline was not 

 quite regular, there being a small secondary increase in sensitivity at 

 about 25 min (early prophase). Henshaw and Cohen (1940) show that 

 there is good agreement between the first peak in sensitivity and changes 

 in viscosity and permeability, but point out that recovery in the egg pro- 

 nucleus up to the time of fusion might also be involved. Lea (1947) also 

 ■ suggests that the early increase in sensitivity is due to a recovery process 

 which he believes can continue up to prophase. Thus the time for 

 recovery decreases as the time after insemination increases. The later 

 drop in sensitivity may be due to irreversible changes leading to division 

 which cannot be affected by radiation; but, as pointed out by Henshaw and 

 Cohen (1940), this explanation fails to account for the secondary peak. 



Henshaw (1940d) has shown that low temperature (0°C) decreases the 

 rate of recovery. Mori d al. (1939) found no effect of dilution of sperm 

 immediately after irradiation, and conclude that failure of the sperm to 

 recover is not due to something produced in the medium by the radiation. 



Recovery of the sort reported for Arbacia apparently does not occur 

 for all invertebrate eggs. Henshaw ct al. (1933) treated Cumingia and 

 Arbacia eggs simultaneously with X rays. The Arbacia eggs showed 

 recovery but the Cumingia eggs did not. It should be noted that recov- 

 ery, in the sense that the egg developed successfully, did occur. The 

 Cumingia eggs simply showed the same delay in first cleavage whether the 

 dose of X rays was given at low intensity over a long time or at high 

 intensity for a brief period. Cook (1939) has also reported no recovery 

 for Ascaris cquorum eggs exposed to X rays and then kept at 5°C for 



