230 THE CONTINUITY OF THE GERM-PLASM AS THE 



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 completed. 



The flaw in this argument lies in the erroneous assumption that 

 the growth of nuclear substance is, when the quality of the nucleus 

 and the conditions of nutrition are equal, unlimited and un- 

 controlled. The intensity of growth must depend upon the quan- 

 tity of nuclear substance with which growth and the phenomena of 

 segmentation commenced. There must be an optimum quantity 

 of nucleoplasm with which the growth of the nucleus proceeds 

 most favourably and rapidly, and this optimum will be represented 

 in the normal size of the segmentation nucleus. Such a size 

 is just sufficient to produce, in a certain time and under certain 

 external conditions, the nuclear substance necessary for the construc- 

 tion of the embryo, and to start the long series of cell-divisions. 

 When the segmentation nucleus is smaller, but large enough to 

 enter upon segmentation, the nuclei of the two first embryonic 

 cells will fall rather more below the normal size, because the 

 growth of the segmentation nucleus during and after division will 

 be less rapid on account of its unusually small size. The succeed- 

 ing generations of nuclei will depart more and more from the 

 normal size in each respective stage, because they do not pass into 

 a resting-stage during embryonic development, but divide again 

 immediately after their formation. Hence nuclear growth would 

 become less vigorous as the nuclei fell more and more below the 

 optimum size, and at last a moment would arrive when they would 

 be unable to divide, or would be at least unable to control the cell- 

 body in such a manner as to lead to its division. 



The first event of importance for embryonic development is the 

 maturation of the egg, i. e. the transformation of the nucleus of the 

 germ-cell into a nuclear spindle and the removal of the ovogenetic 

 nucleoplasm by the separation of polar bodies, or by some ana- 

 logous process. There must be some cause for this separation, and 

 I have already tried to show that it may lie in the quantitative 

 relations which obtain between the two kinds of nucleoplasm con- 

 tained in the nucleus of the egg. I have suggested that the 

 germ-plasm, at first small in quantity, undergoes a gradual incn-asi-. 



