UNION OF THE TWO THEORIES 



403 



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 

 inJietitance, and that chromatin, its essential constituent, is the idio- 

 plasm postulated in NdgelVs theory. This conclusion is now widely 

 accepted and rests upon a basis so firm that it must be regarded as a 

 working hypothesis of high value. To accept it is, however, to reject 

 the theory of germinal localization in so far as it assumes a prelocali- 

 zation of the egg-cytoplasm as a fundamental character of the tgg. 

 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 point 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 there- 

 fore be determined by the paternal chromatin in the germ-nucleus, 

 and not by a predetermination of the egg-cytoplasm. 



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 

 Roux, De Vries, Weismann, and Hertwig ; but all of them may be 

 traced back to Darwin's celebrated hypothesis of pangenesis as a 

 prototype. This hypothesis is so well known as to require but a 

 brief review. Its fundamental postulate assumes that the germ-cells 

 contain innumerable ultra-microscopic organized bodies or geinjnules, 

 each of which is the germ of a cell and determines the development 

 of a similar cell during the ontogeny. The germ-cell is, therefore, 

 in Darwin's words, a microcosm formed of a host of inconceivably 

 minute self-propagating organisms, every one of which predetermines 



