248 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



We arrive at a like conclusion when we consider the quality 

 of the eggs which are produced. The constitution of one species 

 oi Moina contains the cause which determines that each indi- 

 vidual shall produce winter-eggs only, or summer-eggs only ; 

 while in another species the transition from the formation of 

 sexual eggs to the formation of summer-eggs can take place, 

 but only when the winter-egg remains unfertilized. The latter 

 case appears to me to be notably a special adaptation, in this 

 and other species, to the deficiency of males, which is apt to 

 occur. At all events, it is obvious that it is an advantage that 

 an unfertilized sexual egg shall not be lost to the organism. 

 The re-absorption of the winter-egg is an arrangement which, 

 without being the cause, is favourable to the production of 



summer-eggs. 



This subject is by no means a simple one, as is proved by 

 the behaviour of the small group oi Daphnidae. Thus in some 

 species, the winter-eggs are produced by purely sexual females, 

 which never enter upon parthenogenesis ; in others, the sexual 

 females may take the latter course, but only when males are 

 absent ; in others, again, they regularly enter upon partheno- 

 genesis. In my work on Daphnidae, I have attempted to show 

 that their behaviour in this respect is associated with the various 

 external conditions under which the different species live ; and 

 also that the ultimate occurrence of the sexual period, and 

 finally the whole cyclical alternation of sexual and partheno- 

 genetic reproduction, depend upon adaptation to certain ex- 

 ternal conditions of life. 



With the aid of my hypothesis that the egg-nucleus is com- 

 posed of ovogenetic nucleoplasm and germ-plasm, I can now 

 attempt to give an approximate explanation of the nature and 

 origin of the direct causes which determine the production, at 

 one time of parthenogenetic summer-eggs, and at another time 

 of winter-eggs, requiring fertilization. But in such an ex- 

 planation I should also wish to include a consideration of the 

 causes which determine the formation of the nutritive cells of 

 the ^%% and of the sperm-cells to which I have alluded above. 



I believe that the direct cause which determines why the 

 apparently identical cells of the young testis and ovary in the 

 Daphnidae develope in such different directions, is to be found 

 in the fact, that their nuclei possess different histogenetic 



