RADIATION AND ARBACIA GERM CELLS. 463 



had either a short exposure to high intensities, or a longer 

 exposure to low intensities of radiation, as for example, with 

 sperm exposed for 45 to 60 minutes in diffuse daylight. Differ- 

 ential recovery may be produced by radiating eggs intensely for 

 a short period, or less intensely for a longer period (Figs. 31-38). 



Eggs stained with either of the four dyes (1/5,000) and exposed 

 at a distance of 50 cm. for 15 minutes to radiation from a 1,500- 

 watt lamp typically showed evidence of slight recovery in a 

 large proportion of cases. As a rule, with exposures of 30 

 minutes or longer, only about 20 per cent, of the eggs remained 

 alive, and of these a certain proportion (presumably the shaded 

 minority) formed normal plutei. Only a small proportion of the 

 plutei showed evidence of differential recovery after an exposure 

 of 45 minutes. When unfertilized eggs are radiated as above, 

 and are then fertilized by normal sperm, the proportion of 

 abnormal larvae which develop is smaller than in the case of 

 eggs which are fertilized before they are radiated. 



In this series, the photodynamic action of benzoflavine and 

 eosin was less pronounced than that of neutral red and methylene 

 blue. 



Differential Inhibition Followed by Differential Acclimation or 

 Recovery. In experiments with radiation, particularly with ultra- 

 violet radiation, where exposures are exceedingly short (a few 

 seconds to 2 minutes), it is possible to have a primary inhibition 

 followed by a secondary acclimation (Figs. 39-45) or recovery 

 (Figs. 50-52). The oral lobe region recovers more rapidly than 

 do other regions. In any case, the form of the larva indicates 

 that there has been a relatively more rapid return or approach to 

 the normal rate of physiological activity and growth of these 

 highly susceptible regions as compared with less susceptible 

 ones. Consequently the gradient is steepened, and the more 

 apical regions, particularly the oral lobe, are over-developed in 

 relation to the more basal regions, which are under-developed. 



Occasionally forms appear in which more than the normal 

 number of aboral arms are present (Figs. 46-49). Such an over- 

 development of the skeleton is probably a result of differential 

 inhibition persisting after general recovery. (See Child, '16, p. 

 115, and Hinrichs, '26.) 



20 



