FOUNDATION OF A THEORY OF HEREDITY. 243 



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 of Daplmiclae. 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 parthenogenesis. 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 parthenogenetic reproduction, depend 

 upon adaptation to certain external 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 explanation I 

 should also wish to include a consideration of the causes which de- 

 termine the formation of the nutritive cells of the egg 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 

 Daphuidae develope in such different directions, is to be found in the 

 fact, that their nuclei possess different histogenetic nucleoplasms, 

 while, if we neglect individual differences, the germ-plasm remains 

 precisely the same. In the sperm-cells the histogenetic nucleoplasm 

 is spermogenetic, in the egg-cells it is ovogenetic. This must be 

 conceded if our fundamental view is correct, that the specific nature 

 of the cell-body is determined by the nature of its nucleus. 



Similarly, the germ-cells of female Daphnidae, which at first do 

 not exhibit the smallest differences, must really differ in that their 

 nuclei must contain different kinds of nucleoplasm, which are 

 present in different proportions. Germ-cells which are to produce 

 a finely granular, brick-red, winter yolk (Moina rectirostris] must 



li 2 



