432 HEREDITY AND DEVELOPMENT 



tried to form a conception of the ultimate structure of living 

 matter have been led to the assumption— expressed in very varied 

 phraseology — of ultimate protoplasmic units which have the 

 powers of growth and division. It is in no way peculiar to Weis- 

 mann to imagine biophors and to credit them with the powers 

 of growing and dividing. This cannot, indeed, be proved, but 

 many facts point to it. The cell divides, but this is preceded 

 by the division of the nucleus ; the nucleus divides, but this 

 involves splitting of the chromosomes ; and the chromosomes 

 are sometimes visibly composed of still smaller bodies, arranged 

 like beads on a string. As Prof. E. B. Wilson says (1900, p. 84), 

 " Our study of nuclear division reveals to us, not a homogeneous 

 dividing mass, but a descending series of dividing elements, 

 which, as if seen through an inverted telescope, recede from 

 the eye almost to the limits of microscopical vision. There is 

 no reason to place the limit of this series at the point where it 

 vanishes from view, and we are thus almost irresistibly drived 

 to the conclusion that the division of the nuclear substance 

 as a whole must be the result of division on the part of invisible 

 elements, by the aggregation of which the visible structures 

 are formed." Moreover, in many cases the cytoplasm or 

 extra-nuclear part of the cell contains minute bodies or 

 " plastids "—e.g. chlorophyll corpuscles — which also multiply 

 by division. 



Those who find it difficult to believe in the theory that there 

 are multiple sets of analogous determinants in the germ-plasm 

 should consider, for instance, the facts of sex and sexual dimor- 

 phism. A queen bee lays an unfertilised egg which develops 

 into a drone or male, which is in many detailed ways different 

 from the queen, and is primarily different in producing sper- 

 matozoa, not ova. But since this drone has only a mother, no 

 father, there must have been in the fertilised ovum which 

 developed into the mother-bee the potentiality— i.e. the deter- 

 minants — of male reproductive organs and masculine characters. 



