PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 2$? 



must represent different qualities, and second, that the apparatus of 

 mitosis is designed to distribute these qualities, according to a 

 definite law, to the daughter-cells. The particular form in which 

 Roux and Weismann developed this conception has now been gener- 

 ally rejected, and in any form it has some serious difficulties in its 

 way. We cannot assume a precise localization of chromatin-ele- 

 ments in all parts of the nucleus ; for on the one hand a large part 

 of the chromatin may degenerate or be cast out (as in the matu- 

 ration of the egg), and on the other hand in the Protozoa a small 

 fragment of the nucleus is able to regenerate the whole. Neverthe- 

 less, the essential fact remains, as Hertwig, Kolliker, Strasburger, 

 De Vries, and many others have insisted, that in mitotic cell-division 

 the chromatin of the mother-cell is distributed with the most scrupu- 

 lous equality to the nuclei of the daughter-cells, and that in this 

 regard there is a most remarkable contrast between nucleus and 

 cytoplasm. This holds true with such wonderful constancy through- 

 out the series of living forms, from the lowest to the highest, that it 

 must have a deep significance. And while we are not yet in a posi- 

 tion to grasp its full meaning, this contrast points unmistakably to 

 the conclusion that the most essential material handed on by the 

 mother-cell to its progeny is the chromatin, and that this substance 

 therefore has a special significance in inheritance. 



4. The Nucleus in Fertilization 



The foregoing argument receives an overwhelming reinforce- 

 ment from the facts of fertilization. Although the ovum supplies 

 nearly all the cytoplasm for the embryonic body, and the sper- 

 matozoon at most only a trace, the latter is nevertheless as potent 

 in its effect on the offspring as the former. On the other hand, 

 the nuclei contributed by the two germ-cells, though apparently 

 different, become in "the end exactly equivalent in every visible 

 respect in structure, in staining-reactions, and in the number and 

 form of the chromosomes to which each gives rise. But further- 

 more the substance of the two germ-nuclei is distributed with abso- 

 lute equality, certainly to the first two cells of the embryo, and 

 probably to all later-formed cells. The latter conclusion, which 

 long remained a mere surmise, has been rendered nearly a cer- 

 tainty by the remarkable observations of Riickert, Zoja, and Hacker, 

 described in Chapters IV. and VI. The conclusion is irresistible 

 that the specific character of the cell is in the last analysis deter- 

 -mined by that of the nucleus, that is by the chromatin, and that in 

 the equal distribution of paternal and maternal chromatin to all the 

 cells of the offspring we find the physiological explanation of the 



