RADIATION AND ARBACIA GERM CELLS. 



461 



or completely fused in the median line (Figs. 8-16). In these 

 experiments, such forms have been obtained by exposing sperm 

 in relatively great dilutions (1/300 per cent, to 1/6,000 per cent.), 

 as well as with more concentrated sperm (f per cent.), such 

 radiated sperm being then used to fertilize normal eggs. The 

 proportion of differentially inhibited forms for a given dosage is 

 greater when the more dilute sperm is used. The same type of 

 result, i.e., differential inhibition, appears when eggs are radiated 

 before fertilization with normal sperm. In such cases, however, 

 under similar conditions of radiation, the percentage of perma- 

 nently inhibited forms and delayed cleavage is considerably less 

 than in the case of radiated sperm. Radiation of either sex 

 component before fertilization, delays and often interferes with 

 normal cleavage so that gastrulation is physically impossible. 



(See Table I.) 



TABLE I. 



EFFECT OF RADIATION ON CLEAVAGE. 



In many cases exogastrulae are formed (Figs. 17-19). Fre- 

 quently inhibition is carried to the point where the charac- 

 teristics of normal plutei are completely obliterated, and the 

 resulting larva appears as a spherical, apolar mass with non- 

 directive swimming, with or without short skeletal arms im- 

 bedded in the tissue (Figs. 20-24). 



When sperm are exposed for one and a half minutes to ultra- 

 violet radiation, in the case of dilute sperm, or for two and a 

 half minutes, in the case of concentrated sperm, the eggs show 

 80 to 100 per cent, of delayed cleavage, followed by the production 

 of differentially inhibited larvae. (See Table II.) 



1 Figures indicate the percentages of developing eggs which have reached the 

 early blastula stage. The light source was a i.soo-watt bulb, at a distance of 

 25 cm. Sensitizing dyes were used in 1/2,000 concentration. 



