CHAPTER XV 



The Role of the Cytoplasm in Development 

 and Heredity 



o some it may seem that the evidence for the pre- 

 _ ponderating part played by the chromosomes in the 

 transmission of inherited characters is so strong that the 

 cytoplasm must of necessity be regarded as taking a sub- 

 ordinate place, and as constituting the medium in which 

 the chromosomes display their activities rather than having 

 any independent function. A careful survey of the evidence, 

 however, shows that this conclusion is by no means justified. 

 In the first place, the arguments pointing to the primary 

 importance of the chromosomes refer almost entirely to the 

 transmission of characters which show Mendelian segrega- 

 tion, and the suggestion has therefore been made that the 

 factors for Mendelian characters are situatedin chromosomes, 

 while those which do not show Mendelian segregation are 

 borne by the cytoplasm. It is said (for example, by WALKER) 

 that Mendelian characters are varietal, and that racial or 

 specific characters, which are supposed by some not to 

 "mendelize," are borne by the cell as a whole rather than 

 by particular chromosomes. For such a statement as this 

 there is very little real evidence ; it is, in fact, by no means 

 certain that there is any heredity other than Mendelian, for 

 when characters apparently alternative to each other do not 

 segregate in the typical way, this may be due to their being 

 in reality compound; or in crosses between more distantly 

 related forms the absence of segregation may be due to the 

 D. c. 16 



