THE PHYSICAL BASIS OF HEREDITY. i6l 



cells are the remnants of the ends of the chromosomes 

 which have been cast off in the division. In Fig. 6, Z>, 

 the four-celled stage is shown with the karyokinetic 

 figures of the next division. In the lower cells the 

 spindles are seen from the pole, the chromatin is pres- 

 ent in the reduced amount in the form of small granules. 

 In the upper left-hand cell the two full chromosomes 

 are seen, each split longitudinally, while the upper right- 

 hand cell shows a repetition of the reduction phenomenon 

 — viz., the central portion of the two chromosomes, 

 broken up into granules, alone enters into the spindle 

 figure, the outer ends being cast off into the cytoplasm, 

 where they suffer a similar fate to those of the lower 

 cell in the previous division. The next division repeats 

 the process, one cell retaining two full chromosomes, 

 while all the others have the reduced amount. This 

 takes place for five successive divisions and then ceases; 

 from the one cell having the two full chromosomes, 

 the reproductive tissues develop, the others with reduced 

 chromatin form the somatic tissues. Thus is accom- 

 plished a visible structural differentiation of the nuclei of 

 the reproductive cells which distinguishes them sharply 

 from all the somatic tissues in Ascaris. We shall see 

 further on that there is abundant evidence in favour of 

 the theory that the nucleus — i. e., the chromatin — is the 

 bearer of hereditary influences from one generation to 

 the next, and that the specific development and functions 

 of each individual cell are dependent upon the specific 

 changes which take place in the chromatin of its nucleus. 

 In this light the almost isolated case of Ascaris pos- 

 sesses a value and interest that can not be overesti- 

 mated. 



While in the higher forms of animals and plants we 

 find a sharp differentiation of their tissues into somatic 

 and reproductive or germ cells, we must bear in mind 



