394 BIOLOGICAL EFFECTS OF RADIATION 



cleaved and the abscissa represents the duration of exposure in minutes. 

 It will be noted that as the length of exposure increased, the number of 

 diploid cleavages decreased, and the number of haploid cleavages 

 increased. With regard to the percentage of eggs which underwent 

 cleavage, it will be seen that brief exposures and also very long exposures 

 did not prevent eggs from dividing. But at a point midway between 

 these extremes was a period when only about 65 per cent of the eggs 

 divided. 



The question of relative sensitivity of spermatozoa and ova has been 

 considered by Mavor and De Forest (67), using Arhacia, and by H. and M. 

 Langendorff (59, 60) , using Psammechinus. According to the quantitative 

 results of these investigators, retardation in development is always 

 greater when spermatozoa are radiated than when ova are radiated. 

 The cytological basis for this result is not clear. 



The only fertilization studies on higher animals are those of Asdell 

 and Warren (8), who inseminated female rabbits with rabbit spermatozoa 

 which had received large doses of X-rays. Litters of apparently normal 

 young were obtained from two rabbits so inseminated. It is difficult to 

 correlate these results with the work of previous investigators on lower 

 animals. Further studies on mammals would be of value. 



CLEAVAGE 



Probably no other single phase of development has received so much 

 attention as that of cleavage. Dividing egg cells have supphed suitable 

 material, not only for a study of the effect of radiation on cleavage as 

 such, but also for a study of the broader problem of the effect of radiation 

 on cell division in general. It was indicated by the work of early investi- 

 gators and has been shown conclusively by many later workers, that the 

 application of either radium or X-rays in suitable dosages brings about a 

 retardation in the rate at which the fertilized ovum undergoes division. 

 Among the many investigators who have studied this problem have 

 been Bardeen (20), O. Hertwig (52), P. Hertwig (54), G. Hertwig (49 and 

 51), Mottram (68), Richards (81), Packard (72), Richards and Good (83), 

 Seide (86), Hammett, Green, and Davenport (41), and Henshaw (45). 



Henshaw (45) has been able to collect detailed quantitative data which 

 demonstrate the effect of various doses of X-rays on the cleavage of the 

 Arhacia egg. In Henshaw's experiments unfertihzed eggs were radiated 

 and then fertihzed with normal spermatozoa. The time required for the 

 completion of the first cleavage in radiated eggs as compared with normal 

 eggs was carefully recorded. The results show clearly that X-radiation 

 prolongs the time which elapses between insemination and the comple- 

 tion of the first cleavage. The degree of retardation is a function of the 

 amount of radiation administered and also of the time intervening between 

 the moment when radiation begins and the moment when insemination 



