January 17, 1908] 



SCIENCE 



95 



begins to divide. Lillie has shown that 

 differentiation may proceed far in the 

 case of Chcetopterus in the absence of any 

 cleavage. On the whole then, although the 

 lines of cleavage tend to follow the pre- 

 existing lines of differentiation the prin- 

 cipal part played by cleavage in the process 

 of differentiation is in rendering permanent 

 the segregation of the different substances. 

 Finally we may conclude that the nu- 

 cleus plays a less important role in 

 the localization of different substances 

 than in the formation of those substances. 

 Nevertheless, in differentiation, as well as 

 the metabolism, there is every reason to be- 

 lieve that the entire cell is a physiological 

 unit. Neither the nucleus nor the cyto- 

 plasm can exist long independently of the 

 other; differentiations are dependent upon 

 the interaction of these two parts of the 

 cell; the entire germ cell, and not merely 

 the nucleus or cytoplasm, is transformed 

 into the embryo or larva; and it therefore 

 seems necessary to conclude that both nu- 

 cleus and cytoplasm are involved in the 

 mechanism of heredity. 



It is well known that many biologists 

 believe that the nuclei, and more particu- 

 larly the chromosomes of the germ cells, 

 are the exclusive seat of the inheritance 

 material. 0. Hertwig and Strasburger 

 first formulated this hypothesis as a result 

 of their studies on fertilization. Roux sug- 

 gested that the chromatin is the most im- 

 portant part of the nucleus, in view of the 

 exact manner in which it is divided in 

 mitosis. Van Beneden and Boveri discov- 

 ered that the chromosomes come in equal 

 numbers from the egg and the sperm ; that 

 the number of chromosomes in each of the 

 germ cells is one half the number charac- 

 teristic of the species, and that by the union 

 of these cells the characteristic number is 

 reestablished ; and finally that the maternal 



and paternal chromosomes are distributed 

 with exact equality to all the cells of the 

 developing organism. Furthermore, it was 

 shown by Rabl and Boveri that there is a 

 continuity (or persistent individuality) of 

 the chromosomes from one cell cycle to the 

 next. That the nucleus alone is the bearer 

 of the inheritance material was affirmed by 

 Hertwig in 1892, in view of the following 

 facts: (1) The equivalence of the inherit- 

 ance material in male and female; (2) the 

 equal distribution of the inheritance ma- 

 terial to all cells of the organism; (3) the 

 prevention of the summation of the inherit- 

 ance material, by its reduction before fer- 

 tilization; (4) the isotropy of the proto- 

 plasm. 



Since that time many additional evi- 

 dences that the chromatin is the seat of the 

 inheritance material have been brought to 

 light, only a few of which can be summa- 

 rized here. Boveri found in the develop- 

 ment of Ascaris that the germ cells, which 

 preserve all the characteristics of the spe- 

 cies, also preserve all the chromatin of their 

 chromosomes, but that in the body cells, 

 which undergo differentiation, the chromo- 

 somes undergo diminution. Weismann as- 

 sumed as a logical necessity that in the 

 maturation of the egg and sperm there 

 must occur a division of a peculiar type, a 

 reduction division, the significance of which 

 is the halving of the germ plasm and its 

 contained hereditary units, in pi*eparation 

 for the union of egg and sperm in fertiliza- 

 tion. Such a reduction division of the 

 chromosomes has since been observed by 

 many investigators in a large number of 

 organisms. More recently Montgomery, 

 McClung, Paulmier, Wilson and others 

 have found chromosomes of many different 

 sizes and shapes within the same nucleus, 

 and Boveri has shown, by a masterful an- 

 alysis, that in echinids the hereditary value 

 of individual chromosomes is different, al- 

 though here they are all alike in form. 



