THE PROCESS OF FERTILIZATION 291 



any case, the chromosomes, even in their compact rod-like state, 

 consist of two kinds of substance, the chromatin proper, which stains 

 deeply, and the linin, which is difficult to stain ; and it is the latter 

 which, by breaking up, forms the pale part of the nuclear network. 



Thus we can understand that the number of chromosomes remains 

 the same in every cell-generation throughout development, as it is 

 the same in all the individuals of a species. The numbers are known 

 for many species : in some worms there are only two or four 

 chromosomes, while in other related worms there are eight ; in the 

 grasshopper there are twelve, and in a marine worm, Sagitta, eighteen ; 

 in the mouse, the trout, and the lily there are twenty- four ; in some 

 snails thirty-two ; in the sharks thirty-six, and in Artemia, a little 

 salt-water crustacean, 168 chromosomes. In Man the chromosomes are 

 so small that their normal number is not certain — sixteen have been 

 counted. This counting can only be done during the process of 

 nuclear division, for afterwards the chromosomes flow indistinguish- 

 ably together, or rather apart, only to reappear, however, in the old 

 foi'm and number whenever the nucleus again begins to divide. 



It remains to be told what becomes of the centrosphere in cell- 

 division. As soon as the formation of the daughter-nuclei has been 

 brought about by the divergence of the split halves of the loops, the 

 spindle figure begins to retrograde, its threads become pale and 

 gradually disappear, as does the whole radiate halo of the centro- 

 sphere (Fig. F and G). The cell-body has by this time also divided 

 in the equatorial plane of the nuclear spindle, and the centrosome 

 remains usually as a very inconspicuous pale body lying in the cyto- 

 plasm close to the nucleus, reawakening to renewed activity when 

 cell-division is about to recommence (G, csph). 



These, briefly, are the remarkable processes of nuclear division. 

 Their net result is obvious ; the chromatin substance is divided between 

 the daughter-nuclei with the greatest conceivable accuracy. 



It is not so easy to understand the mechanism of this partition, 

 and there are various divergent theories on this point. According to 

 the older idea of Van Beneden, the spindle fibres work like muscles, and 

 by contracting draw the halves of the chromosomes which adhere to 

 them towards the pole, while the rest of the fibres radiating out from 

 the polar corpuscles act as resisting and supporting elements. This 

 view, with many modifications however, has still its champions, 

 and M. Heidenhain in particular has made a notable attempt to 

 establish it and to work it out in detail. Opposed to it stand the 

 views of those who, like 0. Hertwig, Butschli, Hacker, and others 

 regard the rays not as specific elements which were pre-formed in the 



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