541 



lil tiie expei-iment I placed Uaphiiia pulex, embedded iii a drop 

 of ditehwater, immediately on the micaplate of the radium- prepara- 

 tion. In the followijig descriptions I shall designate the pi'eparations 

 used, ('ontaijiing- 0,7 and 3,1 mgi-s. of radium-bromide respectively 

 as capsule A and capsule B. 



Animals belonging to different age-periods were radiated separately. 

 It immediately appeared that the adult Daphnia is much less respon- 

 sive to the radium rajs than the new-born animal. An exposure 

 of 18 hours on capsule A did not affect the animals so as to kill 

 them. If the animal kept alive after this prolonged radiation, it was 

 sometinies seen to succumb later on at the periodical ekdysis of the 

 chitin shell, which was often attended by an abnormal chitin-formation. 

 This then pointed to some injury to the ectoderm. Still, even when 

 the animal remained perfectly healthy, it had become sterile for the 

 rest of its life. In the case of only a few hours radiation without 

 any yolk-rich mature eggs being noticeable in the ovary the repro- 

 ductive faculty is not interfered with : the eggs leaving the ovary 

 later on develop normally and the young generating from these eggs 

 reach sexual maturity in the regular way and produce a healthy 

 offspring. 



However^ iclien radiation takes place, ivhile the ovary bears 

 large yolk-rich eggs, or mhen the eggs have only just entered the 

 broodpouch, a radiation of 25 niinutes on capsule A^ or of some 

 minutes on capsule B, ivill suf fee to cause the eggs to develop ab- 

 normally^ so that they are destroyed already in the blastula-stage 

 and are resorbed in the mother. Susceptibility varies individually, 

 without any apparent connection with the age of the mature females. 

 Whereas the eggs that have oidy just entered the broodpouch are as 

 susceptible as, or more susceptible than before leaving the ovary (abortus 

 sometimes occurs already after I7, minutes' sojourn on capsule B), 

 the resisting power of the embryos is seen to increase during the 

 development. Viable young were developed even after a three hours' 

 radiation on capsule A in the gastrula-stage. Yet in these cases the 

 brood did not seem to be always in good cojidilion, since one of 

 (he 3'onng emerging from such a radiated gastrula — the only one 

 of this lot that leached sexual maturity — produced a brood of 

 an anomalous morphological structure. It appears then, that the 

 future germ-cells of the gastrula in this case must have been injured 

 already during the radiation. 



A greater resisting powei- is displayed by the almost viable young, 

 as they can stand a 20 hours' radiation on capsule A, without any 

 prejudice to their future fecundity. If, however, these young ones 



