DIFFERENTIATION, HEREDITY, SEX 321 



conditions within the organism itself, and as it reacts, as the 

 organism develops step by step, these internal conditions rapidly 

 change. These reactions on the part of the organism fall into 

 two groups. (1) Reactions between the organism (i.e., cyto- 

 plasm and nucleus, whether the organism consists of one cell or 

 many) and its environing stimuli. (2) Reactions between the 

 nucleus and cytoplasm of each cell. The idea of reaction must 

 involve two factors, but while equally necessary for reaction, 

 they are not necessarily of equal value in determining or con- 

 trolling the quality of the reaction. A great many organisms 

 react to light; but the quality of the reaction is determined 

 primarily by the organism. 



The whole structure of the cytoplasm may play a large part 

 in determining the quality of the reactions of the egg, but this 

 cytoplasmic structure is itself the result of a series of interac- 

 tions between cytoplasm and nucleus, and the action of the 

 latter is of primary importance in affecting the quality of the 

 result. Going one step further, what the nucleus does is deter- 

 mined by its structure, and this is also the result of interactions 

 of its parts with one another, and with the cytoplasm, which is 

 its environment; and here again certain elements of the nucleus, 

 namely, the chromosomes, seem to be of primary importance 

 in determining the quality of the interaction. The most impor- 

 tant of the concrete, visible organs of the nucleus are the chro- 

 mosomes. And when we attempt to analyze the behavior of 

 these components we are met by the same problem what 

 determines the structure and behavior of these? Two answers 

 have been offered. First, that here we reach the limit of analy- 

 sis, that the chromosomes are autonomic, self-perpetuating, 

 self-regulating bodies, whose morphology and behavior are the 

 determining factors in all that happens in the life of the organ- 

 ism. Second, that the chromosomes are themselves made up 

 of still more fundamental units, the chromatin granules; that 

 these are the autonomic, self- perpetuating, finally determina- 

 tive units in development, and chromosome structure is the 

 result of the primary activity of these bodies. 



Logically there is no reason why we must stop with the chro- 



