THE CELL AND ITS CONSTITUENT STRUCTURES. 155 



troducing' any hypothetical substance which by assimilation 

 and growth shall produce the body in question. The rapid 

 differentiation, the exact correspondence in position with 

 mathematically determinable lines of force, and the gradual 

 transition of the radiations themselves into the general pro- 

 toplasmic structure of the cell as well as the apparent in- 

 difference as to its cytoplasmic or nuclear or even mixed 

 origin, are to my mind strong arguments in support of the 

 contention that we need not yet fall back on a special sub- 

 stance — a sort of amorphous organ, as the fo7is et origo of 

 the spindle fibres. It is not a little remarkable, however, 

 to observe how very positive many people still seem to be 

 as to the correctness of looking exclusively to either a 

 nuclear, or to a cytoplasmic, or to some special source for 

 the material of which the fibres are made up. 



As we have met with so much variety in the character 

 of the cell structures we have hitherto considered, we might 

 be prepared to find differences extending even to the chro- 

 mosomes themselves. And as a matter of fact we do. It 

 is of course impossible to enter into details here, the subject 

 is much too extensive ; and I purpose only to mention those 

 deviations which seem likely to cast some light on the 

 obscurity which clothes the corporeal organisation which 

 underlies, and perhaps is the cause of, what we term 

 heredity. 



It has become customary to regard the nucleus as the 

 bearer of the hereditary qualities of the organism or of the 

 cell, but the reasoning on which this view is based is not 

 above reproach, for even in fertilisation a certain amount of 

 cytoplasm is imported into the i^^<g along with the male 

 nucleus. 



It is true that since the story of the fusion of male and 

 female centrosomes during fertilisation seems to have become 

 generally discredited, the interest attaching to the cytoplasm 

 has again given way to comparative indifference in so far 

 as questions of heredity are concerned ; and, indeed, what 

 experimental evidence we have appears all to tell in favour 

 of the importance of the nucleus. Boveri's beautiful experi- 

 ments, in which he fertilised non-nucleated fragments of the 



