Peoblems of Gametic Constitution 85 



No sperm is able to develop at all parthenogenetically ; many, possibly all, ova 

 are so able to develop. What is the significance of this from the standpoint of 

 germ-plasm constitution ? 



In the terminology of hereditary reactions the two germ-cells are asserted to 

 be equivalent and to carry essentially the same potentialities or agents. Is this 

 really so ? What evidence is there for this assumption, aside from the pairing 

 of relatively trivial characters ? 



In the crossing of widely divergent types it is, as far as I know, entirely the 

 maternal parent that finds expression in form, symmetry, rates of development, 

 and all general properties ; and only in the special determinative type of action 

 in localized portions that the paternal type shows its influence. In my materials, 

 where I have had, in the crossing of species, many examples of this dominance 

 or prepotency of the female type in widely separated species, the segregations 

 in F2 have shown, not the presence of the paternal factor, but of the maternal 

 factors combined with the paternal determiners, so that the segregated line, like 

 the female parent, was the same as the original stock ; but the segregated male 

 type was quite uniformly not the same as the original type, but differed in con- 

 crete respects. In other words, experience with the materials I have used gives 

 no indication that the male gamete carries any of the general factors of its 

 species, but rather the determiners — that is, the female gamete carries both the 

 essential groups of agents, its specific factors and determiners, while the male 

 gamete carries mainly, if not exclusively, the determiner complex. An egg can 

 develop without fertilization ; a sperm can not ; and may this not be the reason 

 for the decided difference of the two as regards their capacity for development 

 parthenogenetically ? If it be assumed that the two are equivalent in constitu- 

 tion as regards their gametic composition, then why will eggs develop par- 

 thenogenetically and sperms will not? Further, if both are the same in com- 

 position — that is, carry all the gametic agents proper to the species^ — why in 

 widely separated materials is it always the maternal parent that is dominant in 

 form, rates of development, and symmetries, the male having influence only in 

 the determination of special characteristics that may be common to both species ? 

 It seems to me that the question of the equivalent composition of the two 

 gametes has been too commonly assumed to be so, following hypotheses now 

 more or less outgrown. 



Following Weismann and others, we have been prone to think of the germ- 

 plasm as a specific single kind of material with organization into some fixed 

 system, with the different " determiners " occupying in it a specific position, 

 like the side-chains in some complex molecule and its symmetries compared to 

 crystalline organization. My experiences have led to the view that it is not a 

 single substance, but a mixture, or solution of substances which, like many other 

 mixtures of material, shows specific symmetries and special organization char- 

 acteristics. The evidence for this conception comes from experiences with the 

 crossing of different species and the showing that there are different groups of 

 activities, the agents in each group interchanging with similar agents in 

 equivalent groups and with no other. 



The following scheme of composition and classification of the agents in the 

 germ-plasm seems to hold throughout my materials and presents, as far as sym- 

 bolization will permit, the ideas that have been developed with regard to its 



