292 THE EVOLUTION THEOEY 



cell, but as the expression of the orientation of certain protoplasmic 

 particles — an orientation evoked by forces which have their seat 

 within the central corpuscles, and act in the manner of magnetic or 

 electric forces. That the central corpuscles are centres of attraction 

 seems to me hardly open to doubt, and I cannot regard the regular 

 arrangement of the chromosomes in the equatorial plane of the 

 spindle as due to a mere adhesion to contractile threads. Some still 

 unknown forces — chemotactic or otherwise— must be at work here. 

 Later on we shall study the phenomenon of the migration of the 

 sperm-nucleus into the ovum, when it is accompanied by its 

 central body and its halo of rays. Hacker seems to me justified in 

 inferring from this phenomenon alone that the sudden origin of the 

 rays is due to forces resident in the central corpuscle. But un- 

 doubtedly even this ' dynamic ' explanation of karyokinesis is still 

 only at the stage of hypothesis and reasoning from analogy, and is 

 far removed from a definite knowledge of the forces at work. 



For the problems with which we are here chiefly concerned, the 

 problems of heredity, it is enough to know that the cells of multi- 

 cellular organisms possess an extremely complex apparatus for 

 division, whose chief importance lies in the fact that through it the 

 chromatin units of the nucleus are divided into precisely equal parts, 

 and so separated from each other that one half forms one daughter- 

 nucleus, the other half the other. It is not merely that there is an 

 exact division of the whole chromatin in the mass, which could have 

 been effected much more simply, but that there is a regulated distri- 

 bution of the different qualities of the chromatin, as we shall see 

 later. 



It must here be emphasized that the splitting of the chromosomes 

 does not depend on external forces, but on internal ones involved in 

 their organization, and in the definite attractions and repulsions of 

 their component particles which come about in the course of growth. 

 The chromosomes do not split like a trunk that has been broken open 

 with an axe, but rather like a tree burst apart by the frost, that is, by 

 the freezing of the water within itself. I consider it very important 

 that we should recognize this, even though we do not yet know what 

 the forces are that have control in this case, because it leads us to 

 conclude that the structure of the chromosomes is extremely complex, 

 that they are, so to speak, a world in themselves, that they possess an 

 infinitely complex and delicate though invisible organization, in 

 which intrinsic chemico-physical forces produce the regulated succes- 

 sion of changes which we observe. We shall afterwards see that we 

 are led to the same conclusion from another direction— that is, from 



