290 GENERAL EMBRYOLOGY 



tion of the chromatic substance, which in its reactions with the 

 cytoplasm produces new and specific conditions, these lead to' 

 others, and so on through development. 



The conception of the determinative character of the chro- 

 mosomes must now be modified to include the idea that each 

 chromosome is not a simple unit, homogeneous either morpho- 

 logically or physiologically. Each chromosome is to be regarded 

 as made up of a series or group of elements which singly are 

 simple and homogeneous, and behave as physiological units or 

 "determiners." These may or may not correspond with the 

 chromioles, or granules of chromatin, of which the chromosome 

 is composed; and while it is true that they have never been 

 positively identified as units or determiners, some such bodies 

 must be present in the chromosome according to this hypothesis. 

 Such determiners, although apparently necessary hypothetical 

 units cannot be described; they may prove not to be definite 

 particles at all, but rather dynamic relations, or configurations 

 of substance. So for practical and descriptive purposes we are 

 nearly limited to the chromosomes. 



It is quite likely that the chromosomes may not be the only 

 factors in the determination of development, there may be a 

 whole series of factors back of these, and we know that a whole 

 series of factors follows after. But if they are proved to be 

 necessary links in a chain of determining factors, then they are 

 causes of differentiation, and if they are found to be the earliest 

 visible differentiations with which later differentiations some- 

 how correspond, then we may refer to them as the causes of 

 specific differentiation. At some future time it may indeed be 

 possible to push the analysis of the factors of differentiation still 

 farther back; such a possibility is in no wise excluded by the 

 chromosome hypothesis as it stands to-day. 



One of the obvious requirements of any hypothesis of differ- 

 entiation and heredity is that it must readily allow interpreta- 

 tion, in cytological terms, of the enormously complex phe- 

 nomena of alternative or Mendelian heredity. Most of the traits 

 of an organism are the property of the species, common to all 

 the individuals of a specific group. But there are other charac- 



