62 CHARLES PACKARD. 



was irregular. Hertwig states that the abnormalities of the egg 

 chromosomes are produced by the action of the injured sperm 

 chromatin which operates like a poison. Its decomposition 

 products in some way communicate the poison to the sound 

 chromatin. 



There is no evidence, other than that given by Hertwig, that 

 radiated chromatin acts as a poison to the unradiated nucleus. 

 His figures in support of this idea are not convincing; they can be 

 explained more readily on the assumption that the injured chro- 

 matin acts merely as a mechanical hindrance in those instances 

 in which it is actually caught in the cleavage spindle. It is 

 difficult to imagine how disintegrating chromatin can communi- 

 cate its injury to sound chromatin. The idea is apparently 

 drawn from O. Hertwig's theory that the radiation produces a 

 "contagium vivum " which acts in a manner similar to the poison 

 elaborated by a pathogenic bacterium. Such an assumption has 

 no basis in fact and does not explain the observed phenomena. 



The cytological studies of Paula Hertwig on the eggs of Triton 

 and the frog, and of Opperman on the trout, indicate that what- 

 ever effect the radiated chromatin produces in the division of the 

 unradiated nucleus is due to mechanical interference. Such 

 effects can best be seen when the radiated and sound nuclei 

 actually fuse prior to the formation of the chromosomes. The 

 radiated chromatin is eliminated from the spindle and causes a 

 derangement in the spindle if it lies near enough to it. In Chcetop- 

 terus the chromosomes form before the fusion of the nuclei and 

 there is' no evidence that any such interference occurs. 



There is always a noticeable variation in the extent of injury 

 produced by the radiations on a single lot of eggs. The curves 

 shown in the text figure indicate that after the critical length of 

 exposure, when the egg nucleus is greatly injured in the majority 

 of cases, there are still some eggs which develop normally with 

 the diploid number of chromosomes. This variation may be 

 due to the fact that the eggs were not equally exposed, or that 

 some are more resistant than others. The latter is probably the 

 case. Paula Hertwig ('16) showed that when Triton eggs are 

 exposed for a definite period some develop normally and some 

 abnormally. The normal eggs in certain instances showed the 



