THE PHYSICAL BASIS OF HEREDITY. 



43 



make it probable tbat this process is continued with every division, so 

 that ultimately each cell of the adult contains chromosomes, one half 

 of which are the lineal descendants of the chromosomes coming from 

 the father, the other half lineal descendants of the chromosomes coming 

 from the mother. Riickert has found that in a late stage of develop- 

 ment in a crustacean the chromosomes were in two groups, presumably 

 maternal and paternal. Moenkhaus has found that in crossing two 

 species of fishes with structurally 

 and physiologically different chro- 

 mosomes these retained their 

 structural and physiological differ- 

 ences for a number of divisions. 

 From the elaborate provisions to 

 insure the union of the chromatic 

 threads it is quite certain that they, 

 finally, are the carriers of the 

 hereditary power. 



The character of any cell is con- 

 trolled by the nucleus it contains, 

 and, since we have seen that the 

 nucleus contains two different 

 groups of chromosomes, one group 

 containing the peculiarities of the 

 father and the other the peculiari- 

 ties of the mother, the cause of the 

 blending of the two sets of charac- 

 ters in the offspring becomes ap- 

 parent and the greater resemblance 

 in some characters to one parent 

 and in other characters to the other 

 parent may readily be inferred. 



Without attempting to review 

 the recent speculations and observa- 

 tions on the origin of the hereditary 

 cells, I want to give an outline of 

 some observations I made about ten 

 years ago, and which have re- 

 cently been confirmed by Beard. Very early in the development of 

 one of the California viviparous fishes certain cells apparently lose their 

 interest in the development. They undergo very little change, while 

 the rest of the cells are busily engaged in multiplying and forming 

 themselves into the various organs of the young fish. These cells be- 

 come shifted somewhat and probably engage in active migrations. 

 Late in the development they again begin to divide and ultimately give 



Fig. 13. History of the Reproductive 

 Cells in Cymatogaster, from the Begin- 

 ning of One Generation to the Beginning 

 of the Next. 1 Fertilization of the egg. 2. 

 Segmentation of the egg. 3. Segregation 

 of the reproductive cells. 4. Period of in- 

 activity. 5. Multiplication of the cells orig- 

 inally segregated. 6. Time of the differen- 

 tiation of the sexes. 7. Continued multipli- 

 cation of the cells in the female. 8. Period 

 of growth of the individual cells. 9. Period 

 of maturation. 



