FACTORS IX r)XT(V;T:\V 250 



difference is seen in the karyokinetic fio;uro.s. In V\>^. \U\,A, such a 

 two-celled stage is seen from the i)ole; in B, a sliglitly later staple in 

 side view of the spindle. In the upi)er cell of .1, the division is of the 

 usual form, the two chromosomes spht longitudin.tlly, and thrir two 

 halves travel to opposite }>oles of the sijindle (B). Hut in t he lower cell 

 this is not the case. The central portion of the two cliromosomcs 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 clu-omosomes are cast off into the cyto})lasm and are eventually 

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

 fom--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 di\ision. 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 i)henome- 

 non — \iz.,the central portion of the two chromosomes, broken up into 

 granules, alone enters into the spindle figure, the outer ends Ijeing 

 cast off into the cytoplasm, where they suffer a similar fate to those of 

 the lower cell in the previous division. The next division re;)eats the 

 process, one cell retaining the two full chromosomes, while all tlie 

 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 accomplishe<l a visi])le 

 structural differentiation of the nuclei of the rei)roductive cells which 

 distinguishes them shari)ly from all the somatic tissues in Asrnris. 

 We shall see further on that there is abundant evidence in favor of 

 the theory that the nucleus — i. e., the chromatin — is the Ix'arer of 

 hereditary influences from one generation to the next, and that the 

 specific development and functions of each iiKlividunl cell are i\c- 

 pendent upon the specific changes which take place in the chromatin 

 of its nucleus. In this light the almost isolated ea.se of A.scaris pos- 

 sesses a value and interest that cannot Ix' overestimat<'d. 



"While in the higher forms of animals and i)lants we find a sharp 

 differentiation of their tissues into somatic and reproductive or germ 



