542 



leave the brood-pouch during the radiation, they invariably perish, 

 every one of them ; a sojourn on capsule A of a couple of hours 

 only will kill them. After an houi-'s radiation the cardiac action is 

 weakened and irregular and they die soon after. In the bi-ood-pouch 

 they are presumably walled off by the chitin shell of the mothei- 

 and by the fluid in which they are swimming, which protection, 

 however, is not sufficient for the young embryos, which are so much 

 more susceptible to the radium rays. 



A shoi't radiation of a female with maturing egg-cells in the ovary, 

 leads indeed to destruction of the eggs; but it leaves the mother 

 unhurt. Afterwards it even causes sometiniesa more numerous offspring, 

 which phenomenon is analogous to that seen in the action of various 

 poisons on Daphnia pulex ^), on which a small dosage of the poison 

 acts as a stimulus. The resistaiu'e of eggs fi-om one and the same 

 lot sometimes differs very much, as among the eggs that are being 

 destroyed occasionally a single normal young may be seen that 

 does not appear to have suffered at all from the noxious influences 

 that threatened it before the embryonic stage, and later on may 

 possibly possess a normal faculty of reproduction. An anomaly in 

 the structure of such a young occurs oidy rarely. It cannot be said 

 to be typical for i-adium-radiation. Generally these monstra (with 

 abnormal profile or defective intestine) are few and far between. 

 As a rule, therefore, radiation yields a noinial embryonic develop- 

 ment or none at all. This is the reason why I never succeeded yet 

 in breeding mutations of Daphnia pulex by radiation with radium, 

 as Morgan achieved on a large scale with the Drosophila fly. The 

 few abnormal specimens never reached maturity, one excepted, 

 which recovered completely arid produced a normal ofïspring that 

 was still healthy after four months. Indeed my eight years' expe- 

 rience with Daphnia pulex have convinced me that this race though 

 highly modifiable, shows only slight mutability. 



Other researches in experimental embryology also show^ that eggs 

 from the same lot vary as to their resisting power to noxious 

 effects. I here call attention to the reseai-ches of Pearl ") on the 

 difference in degree of resistance evinced by embryos of the domestic 

 fowl to intoxication with alcohol, in which case also the eggs, liable 

 to reach full development bring forth normal chickens. 



It might be objected that in the radiation experiments the eggs 

 are differently exposed. This may occur with a numerous brood, in 

 which the inner eggs are shielded from the noxious rays by the 



1) I.e. p. 1. 



3) Proceedings of the Natioual Acad, of sciences. U. S. A., Vol. II, p. 380. 



