Studies on Chromosomes 35 



only one pair of the chromosomes, should have arisen; yet this 

 is an observed fact. I therefore think the suggestion is worthy 

 of serious consideration. If it could be adopted the necessity 

 of selective fertilization would be avoided, for the observed results 

 would follow from the fertilization of any egg by any spermatozoon. 



But even if in accordance with fact the suggestion is still 

 obviously incapable of direct application to cases in which sex 

 is determined independently of fertilization for instance, sex- 

 production in parthenogenetic development or in hermaphrodites, 

 and in forms (such as Dinophilus) where male-producing and 

 female-producing eggs are distinguishable in size before fertiliza- 

 tion. It is possible that these cases may be explicable (under either 

 general interpretation) as a result of some forms of differential 

 distribution of the chromosomes occurring at the time of the for- 

 mation of the polar bodies (parthenogenesis) or at some earlier 

 period in the cell-lineage of the germ-cells; and this possibility 

 should of course be tested by a close cytological study of the facts. 

 On the other hand, there is nothing in the facts to negative the 

 assumption that in some cases the chromosome-combination, 

 established at fertilization, may be in something like a balanced 

 state that is capable of modification by conditions external to the 

 nucleus (as already suggested in the case of dominance). 



Boveri's interesting observations on the dispermic eggs of 

 Ascaris ('04) have given direct evidence that the chromosomes 

 react to their cytoplasmic surroundings; and the same fact is 

 even more clearly shown by the difference of behavior of the 

 differential chromosomes in the two sexes of Hemiptera during the 

 synaptic and growth-periods. Hence, even though a preestab- 

 lished basis of sex-determination be given in such a physiological 

 dimorphism of the spermatozoa as I have suggested, the sex of 

 the fertilized eggs may in many cases be only a matter of greater 

 or less predisposition and not an immutable predetermination. 

 The nuclei, and hence the primordial germ-cells, may in such 

 cases be in a state of approximate equilibrium, and still retain the 

 power of response to varying conditions in the cellular environ- 

 ment. The production of eggs or spermatozoa in hermaphro- 

 dites may thus be explicable as a result of greater or less nuclear 



