SEGBEGATION. 



11 



cytoplasm, consisting of a semi-liquid ground substance, in which 

 lies the network of the cytoplasmic reticulum, in the meshes of 

 which occur various small bodies called collectively the cytoplasts. 

 Within the nucleus, which is separated from the cytoplasm by the 

 nuclear membrane, are found the chromosomes, which are small 

 bodies of living substance lying in the nuclear sap or ground sub- 

 stance of the nucleus. We must seek for the potentialities of the 

 hereditary characters either in some of these cell organs or in their 

 relations to each other. The behavior of the chromosomes is such 

 as to suggest strongly that they are the seat of at least some of the 

 potentialities in question. The work of Prof. E. B. Wilson and his 

 pupils and others indicates that in certain animals certain identifiable 

 chromosomes are responsible for the differences between the sexes, 

 at least for the primary sexual differences. Several other hereditary 

 characters not directly related to sex behave in such manner as to 

 indicate clearly that they bear to the chromosomes a relation similar 

 to that which sex bears to these cell organs. It is highly probable, 

 therefore, that many hereditary characters depend in some way not 

 yet understood on the chromosomes. In fact, when we describe 

 the known behavior of the chromosomes we describe the known 

 behavior of Mendelian characters. 



It is not necessary in this discussion to consider the various theo- 

 ries regarding the relation of hereditary characters to the organs 

 of the cell. The behavior of the characters studied by Mendel and 

 of hundreds of characters investigated by others leaves no doubt 

 that these characters depend in some way on definite cell organs. 

 This does not necessarily imply that each hereditary peculiarity 

 of a race is represented by a distinct body in the germ cells. A given 

 peculiarity may be due to peculiarities in the composition or the 

 physiological behavior of several cell organs. This much, however, 

 seems to be certain: When two races differ in respect of a character 

 and when the hybrid between these races produces two kinds of 

 reproductive cells, one of which is like the reproductive cells of one 

 of its parents and the other like those of the other parent, as is the 

 case in primroses just cited, then the difference between these 

 two races is due to differences iit a single cell organ or to a group 

 of such organs which act together at all times as if they were insep- 

 arable. In hybridization we are dealing with differences between 

 organisms, and these differences are due to differences between 

 corresponding cell organs in the different races. For instance, sup- 

 pose we have two races of plants which differ only in the fact that one 

 of them has red flowers and the other white and that the hybrid 

 between them produces two kinds of pollen, one of which is identical 

 with the pollen of the red variety and the other with that of the 



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