THE METHOD OF EUGENICS 15 



ready to receive an equal half number from the germ cell 

 with which it unites in fertilization. 



d" 





B 



*>--:-' 



'/)m- 



^■-"^.-.■ 



Fig. 5. — Diagram illustrating the mechanism in the chromatic bodies 

 that secures the segregation of determiners. The determiners are assumed to 

 be packed away in the chromosomes. There are equivalent chromosomes 

 (a' and a", h' and h", etc.) in the nuclei of the male (cf ) and female (9) germ 

 cells that unite in the fertiUzed egg (Fig. 3) and these two sets of chromosomes 

 pass into all the embryonic cells — whether of the soma or germ gland — that 

 develop in the young individual. In the division of ordinary body-cells, as 

 illustrated in Fig. 2, each rod a', a", h', h", etc., splits lengthwise and half 

 of each goes to each daughter cell. But in a division just before the germ cells 

 become ripe, as in Fig. 4C, the like chromosomes unite in pairs as at B. 

 Thus a' unites with a" to form a; h' unites with h" to form b; etc. Conse- 

 quently, the number of chromosomes is reduced to half the tyjiical number. 

 When cell-division thereupon occurs (C) and the chromosomes si)lit, either the 

 chromosomal element that was derived from the father (black) or that de- 

 rived from the mother (white) goes, indifferently, to either daughter cell. 

 Consequently, each germ cell contains some chromosomes of maternal and 

 some of paternal origin but not two chromosomes of the same kind. Since, by 

 hypothesis, each chromo.soino contains particular kinds of determiners it 

 follows that the same germ cell does not contain the (sometimes contnisting) 

 characters of both parents, but some have the paternal character and others 

 the corresponding maternal character. 



