543 



external eggs. But the exposure will presumabl)' be ihe same for 

 the Daphnia moving about tVeelj in the waterdrop on the radium 

 capsule, when, as in our case, there are only 4 or 5 eggs located 

 in the brood-pouch. 



What is the cause of this unequal resistance of the Daphnia eggs? 

 A reduction-division of the chromosomes, which might be res|)onsible 

 for the unequal distribution of paternal and maternal hereditary 

 units does not occur in this parthenogenetic development; the 

 egg retains the number of chromosomes of Ihe mother. A similar 

 difference in the reaction of the eggs to the noxious influences I 

 previously detected, when treating the Daphniae witii phenyl- 

 urethan ^). 



Whereas with a short radiation of a female with mature egg- 

 cells only the first brood succumbs, and the succeeding broods are 

 noi'mal (even very abundant), with a longer radiation also the eggs 

 in a younger stage of development are seen to be damaged, until 

 ultimately the Daphnia becomes completely sterile. After one single 

 radiation a Daphnia may, after many abortus, produce quite unex- 

 pectedly normal young again, if namely the younger oögonia are 

 not damaged. Such an after-effect 1 observed up to the 6^'' of January 

 1917 of a radiation for some hours on capsule A on the 23<^ of 

 November 19J6. x'^fter the first date again young were born that 

 were completely 'normal. 



My experience that eggs of Daphnia pulex in the last stage of 

 maturation are most susceptible to i-adium rays, and that only after 

 a prolonged radiation also the younger egg-cells and at last the 

 oögonia are injured, accounts for Packard's ') experience that after 

 radiation of the Drosophila larvae, the young flies become sterile 

 for some weeks, and afterwards become fertile again. If we bear 

 in mind that results with mammals also favour the theory that 

 especially the mature egg-cells are very susceptible, we are justified 

 in presuming that this holds for the whole animal kingdom. 



How to account for the fact that the maturing eggs are more susceptible 

 to the radium rays than the immature and the remaining cells of 

 embryonic and maternal organism? Again, what molecular trans- 

 formations occur in the protoplasm under the infiuence of radium 

 radiation? The view adopted by several researchers that enzymic 

 actions are accelerated or diverted, prompted me to compare the 

 embryonic development of radiated and non-radiated sisters, which 

 as to temperature and diet had been bred under the same conditions. 



1) I.e. page 1. 



") Journal of exp. Zoology. XIX, p. 832. 



