FACTORS IN ONTOGENY 269 



nucleus being slightly shaded while those of the egg nucleus 

 are black. 



The centrosomc divides together with its aster (D), the two 

 daughter centrosomes move apart to opjjosite poles of the egg, 

 and the tA^^ical ampliiaster of cell division is formed (E), the 

 nuclear membranes disa})i)earing and the chromosomes being 

 drawn together into the equatorial })late where each splits 

 longitudinally. The halves are drawn by the mantle fn)rils 

 toward the opposite poles, and the eg,g divides transversely 

 into two cells (F). This process of division is repeated con- 

 tinuously in each of the resulting generations of cells, and 

 from the mass of cells thus formed develops the new organism. 

 Each cell in the tw^o-celled stage has received half of its 

 chromosomes from the egg nucleus and half from the sperm, 

 thus containing equal amounts from each parent. The centro- 

 some, which, as we have seen, is to be regarded as the dynamic 

 center of the cell division, comes from the spermatozoon alone; 

 the egg, on the other hand, furnishes the yolk and i^ractically 

 all of the cytoplasm. 



After this preliminary outline of the facts of fertilization, we 

 are in a better position to understand the significance of a process 

 which occurs in the development of both egg and sperm cells, 

 namely, the reduction of the chromosomes. The necessity for 

 such a reduction is evident from a moment's reflection. We 

 have seen that the number of chromosomes in the nucleus is 

 a constant and typical one for each animal and plant species 

 so far as known. As fertilization consists in the union of two 

 cells in one, from w^hich the young organism develops, it is 

 plain that, were there no reduction, the number of chromo- 

 somes would be doubled in each succeeding generation. How- 

 ever simple this necessity for reduction may appear, the mi- 

 nutiae of the processes through which it is brought about, and 

 the theoretical significance of these facts, form one of the most 

 involved prol^lems of biology to-day. In a few forms, especially 

 among the lower Crustacea, the facts of the reduction are clear 

 and relatively simple; in other forms they thus far stand in 

 direct contradiction, and, for the present, a comprehensive 

 explanation applicable to all forms must be left to further 

 investigation. 



The significance of reduction turns upon tlie conception 

 of a definite organization and in(hvi(hi:dit \- in tlie diroino- 



