No. 542] SOME ASPECTS OF CYTOLOGY 



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substitution of particular "determiners" or "factors" 

 in the zygote calls forth specific responses that lead to 

 the production of corresponding characters. The reason- 

 ing that applies to the first of these cases seems equally 

 applicable to the second. No one, I suppose, would hold 

 in the first case that the particular molecular groups or 

 "Bausteine" concerned in the change are "bearers of" 

 (i. e., are alone responsible for) the resulting new quali- 

 ties. The qualities of any protein, as Kossel has recently 

 urged, belong to the molecule as a whole, and are not to 

 be regarded as the sum of the qualities of its constituent 

 "Bausteine." Why should we regard in a different light 

 the "determiners" (chemical substances?) concerned in 

 the second case? They are, clearly, not to be regarded 

 as "bearers" or "physical bases" of the characters 

 which depend upon their presence or absence. They are, 

 I repeat, only differential factors of ontogenetic reac- 

 tions that belong to the germ considered as a whole or 

 unit-system. 



In all this I am but expressing what I believe to be the 

 point of view of many recent writers on genetic prob- 

 lems; but what I desire to emphasize is that the prob- 

 lems of cytology should be regarded from the same point 

 of view. It is our task to see whether an apparatus 

 of ontogenetic response can be discovered in the cell that 

 fits with such a conception of the general process of de- 

 termination. Is there cytological evidence of the exist- 

 ence in the germ-cell of such specific factors of reaction 

 as I have referred to— in the nucleus, in the protoplasm, 

 or in both? I think that observation and experiment 

 alike have produced such evidence. Such experiments as 

 those of Boveri on multipolar mitosis, and of Herbst and 

 of Baltzer on the relations of the chromosomes in recip- 

 rocal crosses in sea-urchins have almost conclusively 

 shown that the chromatin does in fact play a causative 

 role in determination. Observation has gradually estab- 

 lished the existence of a complex process of segregation 

 and distribution of the nuclear materials in karyokiiu'sis. 



