Studies on Chromosomes 197 



it. But such an answer implies the existence of a definite indi- 

 vidual genetic relation between the chromosomes of successive cell- 

 generations; and it is this relation, I take it, that forms the essence 

 of the hypothesis of genetic continuity, whether or not we include 

 in the hypothesis the assumption that the chromosomes persist as 

 "individuals" in the resting nucleus where their boundaries seem 

 to disappear. We might, for instance, assume that the chromo- 

 somes are magazines of different substances (e. g., enzymes or the 

 like) that differ more or less in different chromosomes, that are 

 more or less diffused through the nucleus in its vegetative phase, 

 but are again segregated out in the original manner when the 

 chromosomes reform." We have, admittedly, but an imperfect 

 notion of how such a re-segregation may be effected, though the 

 conclusions of Bonnevie already referred to, constitute an impor- 

 tant addition to the earlier ones of Boveri (see '07, p. 232) in this 

 direction. However this may be, in my view the most practicable, 

 indeed the almost necessary, working attitude is to treat the chromo- 

 somes as if they were actually persistent individuals. The facts in 

 Metapodius, which at first sight seem to present so chaotic an 

 aspect, fall at once into order and become intelligible if regarded as 

 due to the presence in the species of a certain number of erratic 

 chromosomes, one or more of which may be introduced into the 

 zygote at the time of fertilization and which in some sense retain their 

 identity throughout the development. The particular combina- 

 tion established at the time of fertihzation is the result of the chance 

 union of two particular gamete combinations. Since the distribu- 

 tion of the supernumeraries to the spermatid nuclei is variable, 

 different gamete combinations occur in the spermatozoa of the 

 same individual; and the same is probably true of the eggs. More- 

 over, adults pf the same species five side by side on the same food- 

 plants and presumably may breed together. Different combinations 

 may thus be produced in the offspring of a single pair, whpther 

 the parents possess the same or different numbers. Metapodius 

 thus fulfills the prediction of Boveri, written nearly twenty years 

 ago. " Wenn bei einer Spezies einmal sehr viele und verschieden- 



^* A view similar to this is suggested by Fick himself in his earher discussion ('05, p. 204), but it 

 does not reappear in Jiis later one. 



