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



viz. that fluctuations occur in the quantity of germ-plasm in 

 the segmentation nucleus ; for there can never be absolute 

 equality in any single part of different individuals. As soon 

 therefore as these fluctuations become so great that partheno- 

 genesis is produced, it may become, by the operation of natural 

 selection, the chief mode of reproduction of the species or of 

 certain generations of the species. In order to place this 

 theory upon a firm basis, we have simply to decide whether 

 the quantity of germ-plasm contained in the segmentation 

 nucleus is the factor which determines development ; although 

 for the present it will be sufficient if we can render this view 

 to some extent probable, and show that it is not in contradiction 

 with established facts. 



At first sight this h3'pothesis seems to encounter serious 

 difficulties. It will be objected that neither the beginning nor 

 the end of embrj'^onic development can possibly depend upon 

 the quantit}' of nucleoplasm in the segmentation nucleus, since 

 the amount may be continually increased by growth ; for it is 

 well known that during embryonic development the nuclear 

 substance increases with astonishing rapidity. By an approxi- 

 mate calculation I found ' that, in the ^g% of a Cyuips, the 

 quantity of nuclear substance present at the time when the 

 blastoderm was about to be formed, and when there were 

 twenty-six nuclei, was even then seven times as great as the 

 quantity which had been contained in the segmentation nucleus. 

 How then can we imagine that embryonic development would 

 ever be arrested from want of nuclear substance, and if such 

 deficiency really acted as an arresting force, how then could 

 development begin at all ? We might suppose that when germ- 

 plasm is present in sufficient quantity to start segmentation, it 

 must also be sufficient to complete the development ; for it 

 grows continuously, and must presumably always possess a 

 power equal to that which it possessed at the beginning, and 

 which was just sufficient to start the process of segmentation. 

 If at each ontogenetic stage, the quantity' of nucleoplasm is 

 just sufficient to produce the following stage, we might well 

 imagine that the whole ontogeny would necessarily be com- 

 pleted. 



' Weismann, ' Beitrage zur Kenntniss der crstcn Entwicklungsvor- 

 gange im Insectenei,' Bonn, 1882, p. 106, 



