1907.] Structural Constituents of the Nucleus, etc. 451 



gymnosperms, they are destroyed so that their structural individuality is lost. 

 That the egg-cytoplasm is essential as building-material is, of course, not 

 disputed, and in so far as it consists of different substances, these will have 

 different prospective values in the final result. In the case of many animal 

 eggs, the cytoplasm shows physical differences in its various regions, and there 

 is reason to think that this diversity in material is, at any rate sometimes, 

 connected with differences in the structures finally to be developed. If, then, 

 the egg be mutilated, so that a definite kind of raw material is abstracted 

 from it, an imperfect embryo might be looked for, unless the removed raw 

 material could be regenerated. The nucleus, as architect, would otherwise 

 only be able to build a body correspondingly defective in certain parts. But 

 such considerations as these do not weaken the position of the nucleus as the 

 agent responsible for the particular direction and control of development 

 which is followed by a cell, or, collectively, by a group of cells. In short, 

 just as we have the strongest grounds for associating secretory and other 

 metabolic activities of the cell with the nucleus as the prime mover, so, also, 

 various lines of evidence indicate the same body, or rather certain con- 

 stituents of it, as guiding the course of those chemical transformations 

 in the cell and in the organism which make themselves apparent in 

 characters such as form, colour, and the like, which can be appreciated by our 

 senses. 



I have indicated the possibility that certain constituents of the nucleus, 

 rather than this body as a whole, may be charged wuth the control and 

 direction of cellular development. It remains therefore to consider the 

 evidence on which such a view, which in some form or another has often been 

 put forward, is based. Darwin, Weismann, De Vries, and others, have each 

 suggested the existence of particles which are responsible for the characters 

 of the individual. And I may at once say that, apart from some such material 

 units, I am unable to understand how the large mass of facts of which we are 

 now in possession can be explained. 



Within recent times our knowledge respecting the results of breeding, and 

 also respecting the details of cellular organisation, has been greatly widened, 

 and I believe that the evidence thus drawn from two entirely different 

 sources irresistibly points to the same conclusion, and that the existence of 

 discrete units which the breeder, on purely theoretical grounds, has been 

 compelled to assume, is entirely borne out by the observed presence of 

 certain structures which exist in vast numbers in the nucleus. These 

 structures enter into the composition of those remarkable bodies, the chromo- 

 somes, which form such striking objects in the nucleus at the periods of 

 division. Of course, I need not say that the particles in question are not to. 



