182 THE PHILOSOPHY OF BIOLOGY 



substance of a definite chemical and physical structure. 

 This is not the germ-cell, nor even the nucleus of the 

 latter, but a certain material contained in the nucleus. 

 The latter contains protein substances containing a 

 greater proportion of phosphoric acid than does the 

 cytoplasm of the cells in general ; these proteins are 

 known as nucleo-proteins, though our knowledge of 

 their chemical structure is, so far, not very exact. 

 It is not, however, these that are the germ-plasm, but 

 a substance in the nucleus which becomes visible when 

 the cell is killed in certain ways, and which becomes 

 stained by certain basic dyes. It is distinguished by 

 this character alone and on that account is loosely 

 called ' chromatin." This substance Weismann 

 identifies as " the material basis of inheritance." 



When a cell divides, a very complex train of events 

 usually occurs. This process of " Mitosis ' exhibits 

 many variations of detail, and without actual demon- 

 stration it is rather difficult to explain clearly. But 

 its essential feature is evidently the exact halving of 

 all the structures in the cell which is about to divide. 

 In the ordinary cell which is not going to divide immedi- 

 ately, the chromatin is diffused throughout the nucleus 

 as very numerous fine granules, recognised only by their 

 staining reactions. They may be concentrated at 

 some part of the nucleus, so that a division through 

 a plane of geometrical symmetry of the cell would 

 not, in general, exactly halve the chromatin. Prior 

 to division, therefore, this substance becomes aggre- 

 gated as granules lying along a convoluted filament 

 of a substance called " linin," which is characterised 

 principally by the fact that it does not stain with the 

 dyes that stain the chromatin. The filament breaks up 

 into short rods, called Chromosomes, and these rods 

 become arranged in the equator of the nucleus. The 



