DIFFERENTIATION, HEREDITY, SEX 289 



morphology appears to be constant and specific at all stages of 

 the individual life history, and through successive generations. 

 Thus the chromosomes at once become the foci of observation 

 and discussion, and the hypothesis of nuclear determination 

 becomes, to a considerable extent, the hypothesis of the specific- 

 ity of the chromosomes. 



This conception has already been outlined in Chapter II; the 

 chromosomes are believed to be differentiated functionally, in 

 a specific manner so that each chromosome of the nucleus 

 represents a center of activity of a particular character. That 

 is to say each chromosome, either individually or as a compo- 

 nent of a unified group, determines a specific form of reaction 

 with the cytoplasm, or rather influences in a particular way 

 certain of the reactions constantly occurring between nucleus 

 and cytoplasm. And the final result of these reactions is the 

 production of certain structural and physiological characteris- 

 tics of the embryo or mature organism. Thus, leaving out all 

 the intermediate chain of processes or reactions, there is an 

 actual correspondence between certain traits of the mature 

 organism and certain chromosomal characters of the gametic 

 nuclei. Of course the chromosomal characters determine only 

 the first step in the development of the corresponding trait; but 

 this in turn determines the next, and so on. And since the 

 quality of one step or reaction in development is determined by 

 the preceding, we are correct in relating directly the character 

 of the final steps in development with the factor that first 

 determined the trend of reaction. 



Emphasis is thus placed upon the physiological character of 

 the relation between chromosome and later structure, and care 

 must be exercised constantly, in the discussion of this subject, 

 to guard against a conception of this relation which is too 

 strictly, morphological, and which might suggest too strongly the 

 conception of development as preformational. A wrong inter- 

 pretation of the modern view of the chromosome relation leads 

 to a rather strict preformational view; but such an idea does 

 not to-day represent the hypothesis fairly. What is formed, 

 or preformed, in the germ is a certain arrangement or configura- 



