FACTORS IN ONTOGENY 259 



difference is seen in the karyokinetic figures. In Fig. 146, A, such a 

 two-celled stage is seen from the pole; in B, a slightly later stage in 

 side view of the spindle. In the upper cell of A , the division is of the 

 usual form, the two chromosomes split longitudinally, and their two 

 halves travel to opposite poles of the spindle (B). But in the lower cell 

 this is not the case. The central portion of the two chromosomes is 

 broken up into a large number of minute chromatin granules which 

 divide, and, as shown in B, form the only portion of the chromosomes 

 drawn up to the poles and entering into the structure of the resting 

 nuclei after the division is complete. The large swollen outer ends of 

 the chromosomes are cast off into the cytoplasm and are eventually 

 absorbed, playing no further part as nuclear structures. C shows the 

 four-celled stage, in which a marked difference in the size of the nuclei 

 of the upper and lower cells is visible. Lying near the margins of the 

 lower cells are the remnants of the ends of the chromosomes which have 

 been cast off in the division. In D 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 present in the re- 

 duced 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 phenome- 

 non 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 the 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 chro- 

 mosomes the reproductive tissues develop, the others with reduced 

 chromatin form the somatic tissues. Thus is accomplished 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 favor 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 de- 

 pendent 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 cannot be overestimated. 



"While in the higher forms of animals and plants we find a sharp 

 differentiation of their tissues into somatic and reproductive or germ 



