THE PHYSICAL BASIS OF HEREDITY. 153 
the other the cytoplasm. In the so-called “resting” 
nucleus—i. e., the nucleus not in active division—the 
chromatin, as we have seen, exists usually in the form 
of scattered granules arranged along the linin network, 
and does not colour readily with nuclear stains (Fig. 
5,4). As division approaches these chromatin gran- 
ules become aggregated together in certain definite © 
areas, forming usually a convoluted thread or skein, 
which now readily takes up the nuclear stains which may 
be used. In some nuclei this skein is in the form of a 
single long filament, in others the chromatin is divided 
up from the first into a series of segments, a condition 
which soon follows in the case of a single filament (Fig. 
5, 8). By transverse fission the latter breaks up into a 
series of segments, the “ chromosomes,” 
the number of which is constant for each 
species of animal or plant. Thus in the 
common mouse there are twenty-four, in the onion 
sixteen, in the sea urchin eighteen, and in certain sharks 
thirty-six. The number may be quite small, as, for ex- 
ample, in Ascaris, a cylindrical parasitic worm inhabiting 
the alimentary canal of the horse. Here the number is 
either two or four, depending upon the variety exam- 
ined. In other forms the number may be so large as 
to render counting exceedingly difficult or impossible. 
In all cases, however, one fact is to be especially noted 
—viz., the number is always an evez one, a striking fact 
which finds its explanation in the phenomena of fertil- 
ization to be discussed later on. 
While the chromatin is collecting into the form of 
the chromosomes the nuclear membrane has disappeared. 
The chromosomes soon reach their maximum staining 
capacity, and appear usually as a collection of rods or 
bands of deeply staining substance lying free in the 
cytoplasm (Fig. 5, C). 
The chromo- 
somes, 
