FERTILIZATION 215 



viduals producing the germ cells or produced by them. From 

 the standpoint of the fertilization process and of heredity, the 

 essential fact is not that the zygote develops into an individual 

 of the same species to which belonged the organisms producing 

 the gametes, for in parthenogenesis, for example, specific organ- 

 isms are produced in the absence of fertilization. The signi- 

 ficant fact here is that offspring may possess some of those 

 characteristics which are the individual possessions of either 

 of the parents. On the whole, offspring inherit, or may inherit, 

 equally from both parents, and such a possibility must depend 

 upon the fact that the zygote is composed of substances or 

 structures derived from both the parent organisms. 



Of course the only substance of the zygote which is derived 

 in equal or approximately equal parts from the two parents is 

 the chromatic portion of its nucleus, and it is frequently said 

 that therefore it must be the nuclear structures of the germ cells 

 which are involved in this fact of equal biparental inheritance. 

 And yet the fact should not be disregarded that the sperm does 

 bring into the egg a certain though indeed a small amount of 

 cytoplasm. The fact that the individual parental characters 

 are inherited equally does not necessarily mean that all non- 

 individual characteristics are thus inherited, for all of the 

 more general species characters are common to both parents, 

 and the offspring might conceivably inherit these wholly from 

 either parent. From this point of view the contribution by the 

 ovum of practically the whole of the cytoplasm of the zygote 

 might have at least two meanings. It might mean that the 

 general species characters of the offspring are determined by 

 the structure of the cytoplasm, and only the individual traits 

 by the nuclear structures; it would follow from this that the 

 spermatozoon takes a relatively subsidiary part in heredity. 

 Or it might mean that the two gametic nuclei are from the 

 beginning equally involved in the determination or direction of 

 development, while the cytoplasm of the ovum merely affords 

 the great bulk of the material basis for this development, and 

 is itself in no wise involved in the qualitative determination of 

 either specific or individual characters of the offspring. It 



