IV.] FOUNDATION OF A THEORY OF HEREDITY. 249 



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 de- 

 termined by the nature of its nucleus. 



Similarly, the germ-cells of female Daphniciac, which at first 

 do not exhibit the smallest difierences, 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 {Muina 

 rectirostris) must possess an ovogenetic nucleoplasm of a some- 

 what different molecular structure from those germ-cells which 

 have only to form a few large blue fat-globules, as in the 

 summer-eggs of the same species. It is further probable that 

 different proportions obtain between germ-plasm and ovo- 

 genetic nucleoplasm, in these two kinds of germ-cells ; and it 

 would be a very simple explanation of the otherwise obscure 

 part played by the food-cells, if we were to suppose that they 

 do not contain any germ-plasm at all, and on this account do 

 not enter upon embryonic development, but are arrested after 

 growing to a certain size. Such an explanation, however 

 would not by itself show why they subsequently undergo 

 gradual solution in the surrounding fluids. But since we know 

 that egg-cells also begin to undergo solution as soon as the 

 parent Daphnid is poorly nourished, we can hardly help also 

 referring the solution of the food-cells to insufficient nourish- 

 ment, occurring as soon as the egg-cell, after the attainment of 

 a certain size, exercises a superior power of assimilation. But 

 hitherto we could not in any way understand why the third out 

 of a group of germ-cells should always gain this superior power 

 and become an egg-cell. If it could be shown that its position 

 is more highly favoured in respect of nutrition, we could under- 

 stand why it outstrips the other three in development, and thus 

 prevents them from further growth. But nothing of the kind 

 can be shown to occur with any degree of probability, as I have 

 previously mentioned in my works on the subject. At that 

 time, having no better explanation, I adopted the view in 

 question, although only as a provisional interpretation, it 

 was not possible for me to seek in the substance of those four 



