FOUNDATION OF A THEORY OF HEREDITY. 237 



I believe I have shown 1 that the summer-eggs are not only de- 

 veloped parthenogenetically, but also that they are never fertil- 

 ized ; 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 egg, 

 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 pur- 

 pose 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 depend, 

 to a certain extent, upon the nutrition of the nucleus, and thus in- 

 directly 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 everywhere 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 differ- 

 ences of nutrition. The phyletic acquisition of the capability of 

 parthenogenetic development must therefore depend upon an alter- 

 ation in the capability of growth possessed by the nucleus of the 



egg 1 - 



This theory of parthenogenesis most nearly approaches Stras- 

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

 rence of parthenogenetic development by the insufficient quantity 

 of nucleoplasm remaining in the egg after the expulsion of polar 

 bodies. The former theory differs 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 segmentation nucleus. Stras- 

 burger assumes that ' specially favourable conditions of nutrition 

 counteract the deficiency of nuclear idioplasm,' while it seems to 

 me that nutrition must be considered as only of secondary import- 

 ance. Thus in bees, as above stated, the same egg may develope 

 parthenogenetically or after fertilization, the nucleus being subject 

 to the same conditions of nutrition in both cases. Strasburger 2 

 1 ' Daphniden,' Abhandlung, vi. p. 324. 2 I.e., p. 150. 



