THEIR SIGNIFICANCE IN HEEEDITY. 367 



we cannot at once assert the impossibility of an early ' reducing 

 division ' on this account, for as I have shown above, the power to 

 develope parthenogenetically depends upon the quantity of germ- 

 plasm contained in the mature egg ; the necessary amount might 

 be produced by growth, quite independently of the number of 

 different kinds of ancestral germ-plasms which form its constituents. 

 The size of a heap of grains may depend upon the number ;of grains, 

 and not upon the number of different kinds of grains. But in 

 another respect such a supposition would lead to an unthinkable 

 conclusion. In the first place, the number of ancestral germ-plasms 

 in the germ-cells would be diminished by one half in each new 

 generation arising by the parthenogenetic method ; thus after ten 

 generations only y^Vf f the original number of ancestral germ- 

 plasms would be present. 



Now, it might be supposed that the ' reducing division ' of the 

 young egg-cells was lost at the time when the parthenogenetic 

 mode of reproduction was assumed by a species ; but this suggestion 

 cannot hold, because there are certain species in which the same 

 eggs can develope either sexually or parthenogenetically (e. g. the 

 bee). It seems to me that such cases distinctly point to the fact 

 that the reduction in the number of ancestral germ-plasms must 

 take place immediately before the commencement of embryonic 

 development, or, in other words, at the time of maturation of the 

 egg. It is only decided at this time whether the egg of the bee 

 is to develope into an embryo by the parthenogenetic or the sexual 

 method ; such decision being brought about, as was shown above, 

 by the fact that only one polar body is expelled in the first case, 

 while two are expelled in the second. But if we are obliged to 

 assume that reproduction by means of fertilization, necessarily 

 implies a reduction to one half of the number of ancestral germ- 

 plasms inherited from the parents, the further conclusion is 

 obvious, that the second division of the egg-nucleus and the expul- 

 sion of the second polar body represent such a reduction, and that 

 this second division of the egg-nucleus is unequal in the sense 

 mentioned above, viz. one half of the ancestral germ-plasms re- 

 mains in the egg-nucleus, the original number being subsequently 

 restored by conjugation with a sperm-nucleus ; while the other 

 half is expelled in the polar body and perishes. 



I may add that observations, so far as they have extended to 



