396 BIOLOGICAL EFFECTS OF RADIATION 



light radiation. Ancel and Vintemberger (6), on the other hand, in 

 their work on the eggs of the frog found no acceleration at all. They hold 

 that radiation causes a retardation whatever the dosage employed. 



One must come to the general conclusion, therefore, that retardation 

 of cleavage after exposure to either radium or X-rays is the rule, and that 

 acceleration is the exception. Light doses may accelerate cleavage; 

 heavy doses always retard cleavage. 



Extensive changes occur in the chromatin of eggs in which cleavage 

 has been retarded by radiation. P. Hertwig (54) and Payne (78), have 

 studied these chromatin changes in detail and have found that the 

 chromosomes of radiated ova break up into granules and present a general 

 disordered appearance. Packard (73) has found in Chaetopterus that the 

 chromosomes which were apparently most seriously injured lacked spindle 

 fibers and exhibited no signs of movement during cleavage. 



GASTRULATION 



As development of radiated individuals progresses, many abnormali- 

 ties often become evident. The stage at which severe abnormalities first 

 appear is, in most cases, that of gastrulation. Gastrulation is a period in 

 development during which extensive cellular movement and cellular 

 differentiation normally occur, resulting in the establishment of the tissues 

 and organs of the new individual. Radiation so disturbs the orderly 

 mechanism of development that the cells appear unable to go through the 

 complex changes incident to gastrulation. Many investigators have 

 observed the irregularities of gastrulation in radiated individuals. Bohn 

 (22, 23) in his early study on the sea-urchin found that exposure of the 

 blastula often prevented gastrulation. If gastrulation had already begun 

 before the exposure, then the gastrula which was formed was often 

 abnormal and short-lived. Perthes (79) found in the case of Ascaris 

 that irregular cell masses developed instead of normal gastrulae and 

 Tur (90, 91) made similar observations on the mollusc, Philine. 



In the case of amphibians, imperfect gastrulation is one of the most 

 commonly observed developmental abnormalities. Schaper (85), and 

 later Hertwig (52), found that gastrulation in radiated amphibians was 

 retarded and often atypical. Bardeen (18, 19, 20), in his extensive 

 observations on toad and frog development, found the effect on gastrula- 

 tion so marked that he divided his larvae into two categories, one in 

 which gastrulation was complete, and the other in which gastrulation 

 was incomplete. Incomplete gastrulation sometimes ran as high as 

 75 per cent in eggs which were radiated during cleavage. In many cases, 

 in which gastrulation appeared to be unaffected, no larval differentiation 

 took place, and always there were large numbers of abnormal larvae. 

 One of the most common results of abnormal gastrulation was the 

 appearance of larvae with spina bifida. In one of Bardeen's experiments 



