302 THEORIES OF INIIERITANCE AND DEVELOPMENT 



gives rise to chromosomes of the same number, form, and size. Van 

 Beneden and Boveri proved (p. 134) that the paternal and maternal 

 nuclear substances are equally distributed to each of the first two 

 cells, and the more recent work of Hacker, Ruckert, Herla, and 

 Zoja establishes a strong probability that this equal distribution con- 

 tinues in the later divisions. Roux pointed out the telling fact that 

 the entire complicated mechanism of mitosis seems designed to effect 

 the most accurate division of the entire nuclear substance in all of 

 its parts, while fission of the cytoplasmic cell-body is in the main a 

 mass-division, and not a meristic division of the individual parts. 

 Again, the complicated processes of maturation show the significant 

 fact that while the greatest pains is taken to prepare the germ-nuclei 

 for their coming union, by rendering them exactly equivalent, the 

 cytoplasm becomes widely different in the two germ-cells and is 

 devoted to entirely different functions. 



It was in the main these considerations that led Hertwig, Stras- 

 burger, Kolliker, and Weismann independently and almost simultane- 

 ously to the conclusion that the nucleus contains the physical basis of 

 inheritance, ajid that chromatin, its essential constituent, is the idio- 

 plasm postulated in Nageli s theory. This conclusion is now widely 

 accepted ; and notwithstanding certain facts which at first sight may 

 seem opposed to it, I believe it rests upon a basis so firm that it may 

 be taken as one of the elementary data of heredity. To accept it is, 

 however, to reject the theory of germinal localization in so far as it 

 assumes a pre -localization of the egg-cytoplasm as a fundamental 

 character of the Qg'g. For if the specific character of the organism be 

 determined by an idioplasm contained in the chromatin, then every 

 characteristic of the cytoplasm must in the long run be determined 

 from the same source. A striking illustration of this fact is given 

 by the phenomena of colour-inheritance in plant-hybrids, as De Vries 

 has pointed out. Pigment is developed in the embryonic cytoplasm, 

 which is derived from the mother-cell ; yet in hybrids it may be 

 inherited from the male through the nucleus of the germ-cell. The 

 specific form of cytoplasmic metabolism by which the pigment is 

 formed must therefore be determined by the paternal chromatin in 

 the germ-nucleus, and not by a pre-determination of the egg-cyto- 

 plasm. 



C. Union of the Two Theories 



We have now to consider the attempts that have been made to 

 transfer the localization-theory from the cytoplasm to the nucleus, 

 and thus to bring it into harmony with the theory of nuclear idio- 

 plasm. These attempts are especially associated with the names of 



