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



fertilization by many spermatozoa ; and, as in these experi- 

 ments, malformation of the embryo would result. In Daplniidae 

 1 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 segmentation nucleus 

 is already formed. 



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

 ^gg» formed during maturation, may either conjugate with the 

 sperm-nucleus, or else if no spermatozoon reaches the egg 

 may, under the stimulus of internal causes, grow to double its 

 size, thus attaining the dimensions of the segmentation nucleus. 

 For our present purpose we may leave out of consideration the 

 fact that in the latter case the individual produced is a male, 

 and in the former case a female. 



It is clear that such an increase in the germ-plasm must de- 

 pend, to a certain extent, upon the nutrition of the nucleus, and 

 thus indirectly upon the body of the egg-cell ; but the increase 

 must chiefly depend upon internal nuclear conditions, viz. upon 

 the capability of growth. We must further assume that the 

 latter condition plays the chief part in the process, for every- 

 where in the organic world the limit of growth depends upon 

 the internal conditions of the growing body, and can only be 

 altered to a small extent by differences of nutrition. The phy- 

 letic acquisition of the capability of parthenogenetic development 

 must therefore depend upon an alteration in the capability of 

 growth possessed by the nucleus of the ^^"g. 



This theory of parthenogenesis most nearly approaches Stras- 

 burger's views upon the subject, for he also explains the non- 

 occurrence of parthenogenetic development by the insufficient 

 quantity of nucleoplasm remaining in the ^%g after the expulsion 

 of polar bodies. The former theory difi'ers however in that the 

 occurrence of parthenogenesis is supposed to be only due to an 

 increase of this nucleoplasm to the normal size of the segmenta- 

 tion nucleus. Strasburger assumes that 'specially favourable 

 conditions of nutrition counteract the deficiency of nuclear idio- 

 plasm,' while it seems to me that nutrition must be considered 

 as only of secondary importance. Thus in bees, as above 

 stated, the same egg may develope parthenogenetically or after 



* * Daphniden,' Abhandlung, vi. p. 324. 



