546 



urchin it is onlj at the close of the blastula-stage that the various 

 properties of tiie chromosomes manifest themselves. 



The period of maturation, in whicii the egg is so extremely 

 susceptible to tiie radium i-ays, also proves to be the critical period 

 for a poison as phenjlnrethan in a certain concentration, as discussed 

 by me in an earlier paper '). 



Just as some eggs of the brood sometimes escape death after radiation 

 with radium, and develop into peifectly normal young, a Daphnia was 



occasionally developed after tieatment with y^^rr n phenylurethan. 



When transmitted to water it produced a normal offspring. This 

 again proves that the resisting power of the eggs lo the danger, 

 threatening them from the outei- world, was occasionally very dif- 

 ferent, even with these parthenogenetic animals. But if they succumb 

 in the struggle, the method of reaction in the two series of experiments 

 is widely different. Whereas with a treatment wilh radium radiation 

 this reaction leads irrev^ocably to degeneration at the close of the 

 blastula-period, a treatment with phenylurethan evolves fully devel- 

 oped monstra, in consequence of a deleterious influence, exerted in 

 the same period of susceptibility. These monsira, however, are not 

 viable after birth: they are not resorbed, but are expelled from the 

 parental organism. 



Summary. 



The egg-cells of Daphnia-pulex are most susceptible to radium 

 radiation in the last stage of maturation. The resisting power in- 

 creases in the embryonic stage. 



In one and the same brood individual differences of susceptibility 

 to the rays of radium is frequently noted. The egg that resists the 

 deleterious influence often develops into a perfectly normal animal, 

 which itself becomes fertile. The rare samples with morphological 

 abnomalies seldom become adults. Only once did we succeed in 

 breeding from such an abnormal young a stock without morpholo- 

 gical anomalies. 



A long-continued radiation from 0,7 mgrs of radium-bromide does 

 not endanger the life of the sexually mature Daphnia, but only its 

 fertility. It depends on the duration of radiation whether only the 

 maturing eggs, the oocytes, or also the oögonia are injured. Large 

 progenies being easy of observation afford an opportunity to study 

 this in every special case. 



1) L. c. p. 1. 



