544 



Up to the moment the eggs left the ovary these sisters were kept in 

 the same culture glass, animals being selected for this experiment 

 whose eggs entered the brood-pouch in the same hour. As soon as 

 the eggs began to develop one Daphnia was radiated for fifteen 

 minutes on capsule A. 



After this the conditions were made ecjual for either animal. In 

 case a normal brood was developed after this radiation, embiyogeny 

 was neither retarded, nor accelerated. At the same hour the heart's 

 pulsation became visible in the brood of the two animals; at the same 

 time the development of I he limbs commenced and the first eye- 

 pigment developed itself ; in the same houi' the young left the parental 

 organism. A similar observation was made at a second radiation. 

 If only the radium action does not pass the physiological boundary 

 there is neither acceleration nor retardation of development. 



Is it the alplia-, the beta-, or Ihe gamma-rays to which the egg- 

 cells of the Daphnia are particularly responsive. In radiating on the 

 capsule with radium-bromide the alpha-rays are screened out by the 

 mica-plate of the capsule, which ihey cannot penetrate and conse- 

 quently I hey do not reach the animal. When separating the Daph- 

 nia from the radium-preparation by a leaden platelet of 3 m.m. 

 thickness, the beta-rays do not reach the Daphnia, while the secon- 

 dary beta-rays are allowed to resorb through a mica-platelet of 50 ;i 

 thickness on which the Daphnia is placed. In I his way Daphniae 

 with maturing eggs in the ovary could stand a ladiation with the 

 gamma-rays from 0,7 mgrs. of radium-bromide for 24 hours, without 

 abortus, which proves the harmlessness of the gamma-rays. When 

 applying the radiumpreparation of 3,1 nigr., which, as has been seen, 

 will destroy the eggs within a few minutes, a radiation of 24 hours 

 with the exclusion of the beta-rays, cotdd be borne without delete- 

 rious influences. In a few of the latter experiments, however, the 

 first brood is aborted. It is, therefore, possible that to a stronger 

 concentration of the gam ma- rays (not obtainable with these prepa- 

 rations) the Daphnia egg-cells prove to be sensitive, a sensitivity, 

 however, that is not to be compared with that to the beta-rays. 



The antagonistic action between uranium and radium, demon- 

 sti'ated by Zwaardemakkr ^j for the frog's heart, induced me to 

 radiate Daphnia pulex in a drop of uranylnitrate and to determine 

 whether resorption of the eggs stayed away in this process. A 

 concentration of 600 mgrs. of uranylniti-ate pro L. is tolerated for 

 some hours without inhibiting the development of the brood ; with 



1) These Proc. XIX p. 1043. 



