But how does this fabric, endowed with an architecture so complicated, actually produce the development of the adult from the egg? The natural mechanism for this purpose is cell division and nuclear division. According to Weismann's supposition--a supposition which forms, as we shall see, a chief corner-stone of his system--there are two kinds of nuclear division, the difference between which has not been observed, but is a corollary from the difference between their results. The one kind is denoted as integral, or doubling division; the other as differential, or differentiating division. The first method has only an incidental importance in Weismann's hypothesis: it consists of the doubling by growth of the rudiments, and of a perfectly fair division of them between the half-chromosomes; it occurs in tissues-cells, where parent-cells divide into daughter-cells exactly similar to each other and to their parents. On the other hand, in differentiating division the rudiments become irregularly grouped during their growth; consequently, on division of the ids, which are composed of determinants, totally different combinations of the determinants are included in the daughter-ids. This method of division of the germplasm plays the chief part in the transformation of the egg into the adult. It has to take place so that the numberless determinants, or guiding particles, of the germplasm may be disentangled and brought forward at the time and place necessary for them to guide the formations of the _determinates_, or independently variable parts of the adult body. To take an example: Weismann's hypothesis requires that when the egg first divides into two, the germplasm should divide into two halves, each containing only one half of the total assemblage of determinants. In each subsequent cell-division this process of segregation is continued, so that the ids, as the phases of embryonic growth occur, contain more and more few different kinds of determinants. Supposing the germplasm to be composed of a million determinants at one stage, in the next it would contain only half a million, and in the next, again, only a quarter-million. In this manner the architecture of the ids becomes simpler and simpler, reaching the simplest conceivable condition in the active cells of the adult body. In these the germplasm consists only of the kind of determinants peculiar to the cells in which they lie; and these determinants are broken up into _biophores_, or bearers of cell qualities.