THE ROLE OF THE CELL ORGANS IN HEREDITY 333 



with other Mendelian characters, whereas others appear to require an 

 explanation of another kind. The results of work in progress at Cornell 

 University on variegated races of maize points in this direction. One 

 of the most interesting problems in cytology and genetics at present is 

 that concerning the manner in which extra-nuclear bodies, such as plas- 

 tids and their primordia, may account for certain types of inheritance, 

 and the extent to which their behavior may be influenced by the 

 nucleus. 



Aleurone Inheritance. We may here refer to the attempt which has 

 been made to explain the inheritance of aleurone color in maize endosperm 

 on the basis of a somatic segregation of special cell organs in the form of 

 granular primordia, which multiply by fission and develop into aleurone 

 bodies of various types and colors. But since aleurone and other endo- 

 sperm characters are inherited in Mendelian fashion, as shown by East 

 and Hayes (1911, 1915), Collins (1911), and Emerson (1918), and since 

 there has been adduced in support of the supposed sorting out of primor- 

 dia no evidence approaching in cogency that upon which the chromosome 

 theory has been built up, geneticists generally are of the opinion that the 

 chromosomes with their well known mechanism of segregation offer the 

 best promise of an explanation of the inheritance of aleurone characters, 

 though all admit that other organs may play a part in bringing these 

 characters to expression. Furthermore, the case for the self-perpetuity 

 of the aleurone grain is much weakened by the fact of their artificial pro- 

 duction by Thompson (1912). 



The theory that chondriosomes are concerned in heredity has been 

 discussed in Chapters VI and XII. 



General Conclusions. In conclusion the statement may again be 

 made that as genetical researches multiply it becomes increasingly clear 

 that the characters in which an individual resembles that from which it 

 sprang are not in every case transmitted to it in the same manner. Those 

 characters which are inherited according to Mendelian rules, to anticipate 

 a conclusion based on evidence to be presented in the next chapter, in all 

 probability owe their repeated appearance in successive generations to 

 "factors" of some sort which are transmitted by the chromosomes of 

 the nucleus. This applies also to those characters which, while Men- 

 delian in distribution, depend for their expression upon the presence 

 of other cell organs (plastids) which may have an individuality of their 

 own. 



All or nearly all of the hereditary contribution made by the male 

 parent must in most organisms be in the above form, since the male 

 gamete consists almost exclusively of nuclear material. The female 

 gamete, or egg, in addition to the clearly Mendelian characters repre- 

 sented by factors in its nucleus, may at least in the case of many animals 



