545 



a higher concentration tiie Dapliina itself snccumbs in less time. 

 When a Daphnia with niatnring eggs in the ovary is placed in a 

 solution of 500 mgrnis of nranylniti'ate pro L. anil after half an 

 hour again in a drop of the same tlnid on the radinin capsnle of 

 3,1 mgrs, the radial ion nuw be continned for ^ to 4 honrs in a 

 series of experiments withont causing the brood to abort, while, 

 under the same circumstances, the eggs, when phiced in water, are 

 fatally injured already after some minutes. Sometimes, however, the 

 protecting influence of uranylnitrate was not at all discernible. Up 

 to the present I have not been in a position to acconnt for tliese 

 various results. This also applies to the lower concentration of 

 uianylnitrate. 



More than (Iftj Daphniae were examined in microscopic sections 

 and compared with non-radiated specimens. Normal maturation of 

 eggs in Daphniae has already been described by Kühn '). Broadly speaking 

 my findings for the normal eggs are in agreement with his. A prolonged 

 radiation did not enable me to detect in the maturing eggs a,ny 

 change either in the chromosomes, or in the nuclear body, or in 

 the egg-plasma. Only in one polar spindle (for the formation of the 

 first polar body) was the number of chromatin rods larger than 

 could be anticipated with twice the number of chromosomes. 

 Any possible alteration in the shape of the chromosomes is difficult 

 to detect owing to the small dimensions. 



Not before the blastula-stage, that is about the time when also 

 in the living animal under the low-power microscope the embryos 

 are seen to succumb, well-marked alterations take place in the 

 nuclei, characterised by a collapse of the chromatin into coarse 

 granules. The injury to the eggs, however, has been done long 

 before the aided eye can detect it. 



Though microscopic examination did not put us in a position to 

 ascertain whether the noxious action of radium-rays has initially 

 affected the nucleus, the cell- plasma or both, the high degree of 

 susceptibility of the egg in a period when also considerable evolu- 

 tions take place in the nucleus (formation of the polar spindle and 

 decomposition of the large nuclear body) is indicative of a noxious 

 effect of the beta-rays, especially on the nucleus. The fact that the 

 first cleavage proceeds regularly and only at the close of it degene- 

 ration manifests itself, may be explained, when we call to mind 

 BovERi's '*) investigations, which demonstrated that with the Sea- 



1) Arch. f. Zellforschung, Vol. I, p. 538. 

 «) Jenaische Zeitschr. Vol. 43, 1907. 



