8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 93 



The radiotoxic effect on the ova of both species was exhibited in 

 different ways. On the ova of both Toxocara and Toxascaris it was 

 apparent in some cases that the uhraviolet Hght had some direct and 

 undelayed lethal effect, inasmuch as many eggs never underwent any 

 cleavage. On the other hand, some of the ova of both species developed 

 in part until development was definitely arrested. In order to check 

 on this point, the culture in dish i (wave length 3022 A) was re- 

 counted at the expiration of 21 days in an effort to determine whether 

 any ova which were in intermediate stages of development at the time 

 of the first count. 11 days after exposure, had completed their develop- 

 ment. It was found that none of the ova which were partially developed 

 at the time of the first count had completed their development at the 

 time of the second count 10 days later. It would appear that such ova 

 had been definitely and permanently arrested in development, a cir- 

 cumstance which agrees with the results obtained by Stevens (1909), 

 who believed that such ova may be said to be paralyzed to such an 

 extent that they are unable to initiate any further mitosis. 



In some of the ova arrested in the course of development it was 

 apparent that cleavage had proceeded normally up to a certain point. 

 On the other hand, all sorts of irregularities were noted in the develop- 

 mental stages of some of these eggs. These irregularities were most 

 frequent in Toxocara 0x2.. In some the cytoplasm was apparently 

 degenerated and was distributed in various areas within the shell. In 

 other ova the cytoplasm contained large vacuoles. Irregularities and 

 abnormalities in blastulation and gastrulation were marked. Some of 

 the Toxocara ova contained partly formed embryos., the unformed 

 remainders of which were composed of undiff'erentiated and irregularly 

 formed masses of cells. In some of the eggs the first somatic stem 

 cell failed to develop, whereas the first germinal stem cell divided 

 many times. In other eggs the first germinal stem cell developed only 

 partly. In most of the Toxascaris ova that showed development it 

 would appear that segmentation proceeded normally up to the 8- to 

 i6-cell stage, at which stage it was definitely arrested. Development 

 in none of these ova proceeded to the blastula or gastrula stage, except 

 in a ^'ery few eggs which actually reached embryonation. In all ova 

 of both species which became embryonated, the larvae appear to have 

 ensheathed normally. 



In order to determine the infectivity of embryos which developed 

 in the irradiated ova, on August 11, 1934, the three cultures irradiated 

 in series C were each fed to a young albino rat ; at the same time the 

 control culture was fed to a rat from the same litter. The rat which 

 received the control culture gave birth to a litter of 10 young on August 



