358 CLASSICS OF MODERN SCIENCE 



But does it therefore follow that the quantity of germ-plasm in the 

 segmentation nucleus is not the factor which determines the beginning 

 of embryonic development ? I believe not. It can be very well imag- 

 ined that the nucleus of the egg, having expelled the ovogenetic 

 nucleoplasm, may be increased to the size requisite for the segmenta- 

 tion nucleus in one of two ways : either by conjugation with a sperm- 

 nucleus, or by simply growing to double its size. There is nothing 

 improbable in this latter assumption, and one is even inclined to in- 

 quire why such growth does not take place in all unfertilized eggs. 

 The true answer to this question must be that nature pursues the sex- 

 ual method of reproduction, and that the only way in which the gen- 

 eral occurrence of parthenogenesis could be prevented was by the pro- 

 duction of eggs which remained sterile unless they were fertilized. 

 This was effected by a loss of the capability of growth on the part of 

 the egg-nucleus after it had expelled the ovogenetic nucleoplasm. 



The case of the bee proves in a very striking manner that the dif- 

 ference between eggs which require fertilization, and those which do 

 not, is not produced until after the maturation of the egg and the 

 removal of the ovogenetic nucleoplasm. The increase in the quantity 

 of the germ-plasm cannot have taken place at any earlier period, or 

 else the nucleus of the egg would always start embryonic develop- 

 ment by itself, and the egg would probably be incapable of fertiliza- 

 tion. For the relation between egg-nucleus and sperm-nucleus is 

 obviously based upon the fact that each of them is insufficient by it- 

 self, and requires completion. If such completion had taken place 

 at an early stage the egg-nucleus would either cease to exercise any 

 attractive force upon the sperm-nucleus, or else conjugation would be 

 effected, as in Fol's interesting experiments upon fertilization by 

 many spermatozoa; and, as in these experiments, malformation of the 

 embryo would result. In Daphnidae I believe I have shown that the 

 summer eggs are not only developed parthenogenetically, but also 

 that they are never fertilized ; and the explanation of this incapacity 

 for fertilization may perhaps be found in the fact that their segmenta- 

 tion nucleus is already formed. 



We may therefore conclude that, in bees, the nucleus of the egg, 

 formed during maturation, may either conjugate with the sperm- 

 nucleus, or else if no spermatozoon reaches it the egg may, under the 



