58 THE PHYSICAL BASIS OF INHERITANCE 



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

 nucleus only ? 



A Cautious Acceptance of the Conclusion that the Germ- 

 nuclei are the Bearers of the Hereditary Qualities. — Many 

 observations 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 and general acceptance that the 

 nucleus is the chief or exclusive bearer of the hereditary qualities, 

 and as this is important in connection with some developments 

 of the theory of heredity, we must give some attention to the 

 basis on which the conclusion rests. 



1. Argument from cell-division. — Roux, Hertwig, Kolliker, 

 Strasburger, 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 scrupulous equality " to form the basis of the nuclei of the 

 daaghter-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 sub- 

 stance, therefore, has a special significance in inheritance." 



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 are rendered precisely equal 



