352 CLASSICS OF MODERN SCIENCE 

 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 segmenta- 

 tion, 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 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 uncontrolled. The 

 intensity of growth must depend upon the quantity of nuclear sub- 

 stance with which growth and the phenomena of segmentation com- 

 menced. 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 seg- 

 mentation nucleus. Such a size is just sufficient to produce, in a 

 certain time and under certain external conditions, the nuclear sub- 

 stance necessary 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 imme- 

 diately 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. c, 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 



