multiplication through division, a whole multicellular organism of a particular structure and definite differentiation, in short, a new individual of the particular species to which the parents belong. We call the substance of which these chromatin granules consist by the name first introduced into science by Nägeli, though only to designate a postulated substance which had not at that time been observed, but which he imagined to be contained within the cell-body--by the name _Idioplasm_, that is to say, a living substance determining the individual nature (εἶδος = form). I am anticipating here, and I reserve a more detailed explanation until I can gradually bring together all the facts which justify the conception I have just indicated of the 'chromatin grains' as an 'idioplasm,' or, as we may also call it, a 'hereditary substance.' That this chromatin must be something quite special we see from the processes of cell and nuclear division, which I shall now briefly describe. [Illustration: FIG. 74. Diagram of nuclear division, adapted from E. B. Wilson. _A_, resting cell with cell-substance (_zk_), centrosphere (_csph_) which contains two centrosomes, nucleolus (_kk_); and chromosomes (_chr_), the last distributed in the nuclear reticulum. _B_, the chromatin united in a coiled thread; the centrosphere divided into two and giving off rays which unite the halves. _C_, the nuclear spindle (_ksp_) formed, the rays more strongly developed, the nuclear membrane (_km_) in process of dissolution, the chromatin thread divided into eight similar pieces (_chrs_), the rays are attaching themselves to the chromosomes. _D_, perfected nuclear spindle with the two centrospheres at the poles (_csph_) and the eight chromosomes (_chrs_) in the equator of the spindle, all now longitudinally split. _E_, daughter-chromosomes diverging from one another, but still united by filaments, the centrosomes (_cs_) are already doubled for the next division. _F_, daughter-chromosomes, quite separated from one another, are already beginning to give off processes; the cell-substance is beginning to be constricted. _G_, end of the process of division: two daughter-cells (_tz_) with similar nuclear reticulum (_tk_) and centrospheres (_csph_), as in _A_.] When a cell is on the eve of dividing we observe first that the chromatin grains, which have till then been scattered throughout the network of the nucleus, approach each other and arrange themselves into a long thin thread which, irregularly intertwined, forms a loose skein, the so-called coil-stage (Fig. 74, _B_). The thread then begins to thicken, and somewhat later it can be seen to have broken up into a number of pieces of equal length, as if it had been cut into equal pieces with scissors (_C_).