IV.] FOUNDATION OF A THEORY OF HEREDITY. 235 



The flaw in this argument Hes 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 

 uncontrolled. The intensity of growth must depend upon the 

 quantity of nuclear substance with which growth and the pheno- 

 mena 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 neces- 

 sary for the construction 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 succeeding 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 opti- 

 mum 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 ^gg^ 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 analogous 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 contained in the nucleus of the ^g%. I 

 have suggested that the germ-plasm, at first small in quantity, 

 undergoes a gradual increase, so that it can finally oppose the 

 ovogenetic nucleoplasm. I will not further elaborate this sug- 

 gestion, for the ascertained facts are insufficient for the purpose. 

 But the appearances witnessed in nuclear division indicate that 

 there are opposing forces, and that such a contest is the motive 



