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THE AMERICAN NATURALIST [Vol. XLVII 



These experiments of Hertwig also afford interesting 

 data as to the nature and importance of the part played 

 by the chromatin in development. The cells of the em- 

 bryos which resulted from eggs fertilized by intensively 

 treated sperm were found by 0. Hertwig, P. Hertwig, and 

 Poll, to contain the reduced number of chromosomes 

 showing that the paternal chromatin had been destroyed 

 by the treatment. Griinther Hertwig found the nucleus 

 volume in radium larvsB to be one half the size of the 

 nucleus in the control; he measured the mass of nuclei 

 of nerve cells, liver cells, blood corpuscles, embryonic 

 muscle cells, etc. The entire larva was smaller. P. Hert- 

 wig found the male pronucleus derived from intensively 

 treated sperm to be modified in the first and second divi- 

 sions of the frog's egg and Opperman found the same in 

 the trout. O. Hertwig found in Triton eggs that the in- 

 tensively radiumized male chromatin took no part in the 

 developmental process and the soma cells contained one 

 half the chromosome number. The male chromosome set 

 falls out of the development and the soma nuclei contain 

 only the female set. 



Finally, Hertwig obtained another most striking result 

 which may be mentioned, although it is not entirely in 

 line with the present subject. When eggs instead of the 

 spermatozoa were subjected to intensive treatments of 

 2 to 5 hours with radium, the chromatin of the female 

 pronucleus was found to be broken down and destroyed. 

 If eggs, after such intensive treatment, were fertilized by 

 normal sperm, it was found that they developed almost 

 normally, although when eggs were treated from 15 

 minutes to -J hour they always developed abnormally 

 though fertilized with normal spermatozoa. Hertwig, 

 therefore, concludes that the intensively treated eggs 

 fertilized by normal sperm develop by the process of 

 merogony ; that is, the egg nucleus being destroyed by the 

 treatment, the sperm nucleus enters the egg and causes 

 development to proceed in the same way that the female 

 pronucleus acts in parthenogenesis. Only one set of chro- 



