232 COXTIiVUITY OF THE GERM-PLASM AS THE [IV. 



that changes arise in the constitution of the nucleoplasm at 

 each cell-division which takes place during the building-up of 

 the embryo, changes which either correspond or dilfer in the 

 two halves of each nucleus. If, for the present, we neglect the 

 minute amount of unchanged germ-plasm which is reserved 

 for the formation of the germ-cells, it is clear that a great many 

 different stages in the development of somatic nucleoplasm are 

 thus formed, which may be denominated as stages i, 2, 3, 4, &;c., 

 up to n. In each of these stages the cells differ more as de- 

 velopment proceeds, and as the number by which the stage is 

 denominated becomes higher. Thus, for instance, the two first 

 segmentation spheres would represent the first stage of somatic 

 nucleoplasm, a stage which may be considered as but slightly 

 different in its molecular structure from the nucleoplasm of the 

 segmentation nucleus ; the first four segmentation spheres 

 would represent the second stage ; the succeeding eight spheres 

 the third, and so on. It is clear that at each successive stage 

 the molecular structure of the nucleoplasm must be further 

 removed from that of the germ-plasm, and that, at the same 

 time, the cells of each successive stage must also diverge more 

 widely among themselves in the molecular structure of their 

 nucleoplasm. Early in development each cell must possess its 

 own peculiar nucleoplasm, for the further course of develop- 

 ment is peculiar to each cell. It is only in the later stages that 

 equivalent or nearly equivalent cells are formed in large num- 

 bers, cells in which we must also suppose the existence of 

 equivalent nucleoplasm. 



If we may assume that a certain amount of germ-plasm must 

 be contained in the segmentation nucleus in order to complete 

 the whole process of the ontogenetic differentiation of this sub- 

 stance ; if we may further assume that the quantity of germ- 

 plasm in the segmentation nucleus varies in different cases : 

 then we should be able to understand why one egg can only 

 develope after fertilization, while another can begin its develop- 

 ment without fertilization, but cannot finish it, and why a third 

 is even able to complete its development. We should also 

 understand why one egg only passes through the first stages of 

 segmentation and is then arrested, while another reaches a few 

 more stages in advance, and a third developes so far that the 

 embryo is nearly completely formed. These differences would 



