INTRODUCTION 



31 



of the entire organism in each phase of its development, at any 

 rate as long as they are capable of multiplying, and are pos- 

 sessed of vitality. The chromatin in all the cells of the body is 

 derived from that in the fertilised ovum, while the development 

 of the body from the egg-cell is effected by a series of cell- 

 divisions, each of which includes a division of the nucleus in 

 the manner just described. In the process of ontogeny the 

 chromatin of the first nucleus undergoes repeated subdivisions 

 into two parts of equal volume, and it would very soon become 

 so small as to be invisible even under the highest powers of the 

 microscope, if it did not continue to grow, as does the cell-body. 

 This occurs just as much in the case of numerous animal eggs 

 to which no nutrient material is supplied during the develop- 

 ment of the embryo, as in that of those which are nourished 

 from the beginning, or of plants which as a rule begin to obtain 

 their own nutriment at a very early stage. The chromatin, or 

 hereditary substance of the fertilised ovum, enters upon a long 

 and complex process of growth, which only ceases when no 

 new cells are produced either for the formation of new parts, or 

 to replace old ones, — that is to say, at the end of the life of the 

 individual. This growing hereditary substance may be com- 

 pared to a tree whose branches arise in strict dichotomy, except 

 for the fact that the chromatin does not consist of one continu- 

 ous mass, but of a number of separate particles not actually 

 contiguous with one another ; for at each cell-division the two 

 halves of the chromatin rods separate never to unite again in 

 one nucleus. Each is finally contained in a special nucleus, 

 which is separated from the rest by being enclosed in a special 

 cell-body. The question now arises as to whether all these 

 fragments of the hereditary substance which compose the chro- 

 matin 'tree' of an organism are similar to, or different from, 

 one another, and it can easily be shown that the latter must 

 be the case. 



In order to prove this, we take as our basis the well-grounded 

 assumption that the chromatin in the nucleus of the fertilised 

 egg is the substance on which heredity depends. Thus we 

 know tha,t the possibility of the offspring resembling its father, for 

 example, in a thousand different physical and mental characters 

 depends on the minute mass of a few chromatin granules in 

 the nucleus of the sperm-cell, and that the characters of a fully 

 formed organism depend as a whole, as well as in detail, on the 



