CHAPTER XIV 



THE ROLE OF THE CELL ORGANS IN HEREDITY 



The chief interest of cytology at the present time probably lies in the 

 relation which it bears to the subject of heredity. From the time when 

 the problems of cell research first began to take definite shape, especially 

 since a connection between the activities of the cell and the phenomena 

 of inheritance was suggested, the efforts of most cytologists have con- 

 tributed directly or indirectly to the solution of two great and closely 

 interrelated problems of biology: the problem of ontogenetic develop- 

 ment and the problem of heredity. The aid which cytology has afforded 

 in these respects has been invaluable. Not only has it been able to 

 discover a large number of the significant facts of individual development, 

 or ontogeny, but it has also thrown a flood of light upon many obscure 

 matters in the field of heredity, and has so come to be an important factor 

 in the study of phylogeny and evolution. 



The Law of Genetic Continuity. — "The most fundamental contribu- 

 tion of cell-research to the theory of heredity," says Wilson (1909), "is 

 the law of genetic continuity by cell-division. Cells arise only by the 

 division of preexisting cells ... In each generation the germinal stuff 

 runs through the same series of transformations; hence that reappearance 

 of the same traits in successive generations that we call heredity." 



It is by the light of the above law that we are enabled to see some- 

 thing of the nature of the material continuity which exists between suc- 

 cessive stages of the ontogenetic development, and also between success- 

 ive generations. It is to be remembered, in the first place, that all the 

 cells of the adult multicellular organism are derived by repeated division 

 from the single cell (ordinarily a zygote or a spore) with which develop- 

 ment starts, so that the causes of events occurring at any particular stage 

 are to be sought largely in the reactions of cells at earlier stages; and, in 

 the second place, that the material link connecting two successive gen- 

 erations is a single organized cell, usually a gamete or a spore, which 

 means that the heritage of a long ancestry is in some way represented in 

 this single cell and its capabilities. "The conception that then 1 is an 

 unbroken continuity of germinal substance between all living organisms, 

 and that the egg and the sperm are endowed with an inherited organiza- 

 tion of great complexity, has become the basis for all current theories of 

 heredity and development" (Locy, 1915, p. 224). 



