58 THE PHYSICAL BASIS OF INHERITANCE 



containing unspecialised germinal material. In similar con- 

 ditions similar material produces similar results. 



But, if this has become clear, we have now to inquire into 

 the precise nature of the physical basis which conserves the 

 heritable qualities. Is it the germ-cell as a whole that is 

 essential, or is the cytoplasm most important, or is it the 

 nucleus only ? 



Importance of the Chromosomes of the Germ-nuclei. — Many observa- 

 tions go to show that the nucleus of a cell plays an important part 

 in nutritive and constructive processes, and it is certain that a cell 

 artificially bereft of its nucleus will soon die if left to itself. The 

 nuclear material (karyoplasm or nucleoplasm) is an essential part of 

 the vital organisation. The view has gained ground that the 

 chromatin bodies or chromosomes are the chief, if not the exclusive, 

 vehicles of the hereditary qualities. 



Let us consider some of the arguments in support of this 

 view. 



i. Argument from cell-division. — Roux, Hcrtwig, Kolliker, Stras- 

 burger, and many others, have emphasised the fact that, in the 

 ordinary (mitotic) form of cell-division, the chromatin or readily 

 stainable material of the nucleus is divided " with the most scrupu- 

 lous equality " to form the basis of the nuclei of the daughter-cells, 

 while the cytoplasm or general cell-substance " undergoes on the 

 whole a mass-division — a most remarkable contrast." As Prof. 

 Wilson says (1900, p. 351) : " This holds true with such wonderful 

 constancy throughout the series of living forms, from the lowest to 

 the highest, that it must have a deep significance. And while we 

 are not yet in a position to grasp its full meaning, this contrast 

 [between nuclear and cytoplasmic behaviour in division] points 

 unmistakably to the conclusion that the most essential material 

 handed on by the mother-cell to its progeny is the chromatin, and 

 that this substance, therefore, has a special significance in in- 

 heritance." 



2. Argument from maturation. — In the changes which lead up to 

 the ripe egg and the fully-formed spermatozoon, there is, as we have 

 seen, an elaborate preparation whereby the germ-nuclei which unite 

 in fertilisation arc rendered precisely equal as regards the number of 

 their chromosomes. On the other hand, the cytoplasm of the 

 relatively large, passive, often food-laden and ensheathed ripe ovum 



