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, A}. 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, J3). By transverse fission the latter breaks up into a 



series of segments, the " chromosomes," 

 ' chrom - 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 Asian's, 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 even 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). 



