KARYOKINESIS 483 



spindle, so that we have, in all, 48 chromosomes, 24 at each pole. 

 This completes the second and third phases of division. 



The fourth or telophase consists of retrogressive changes, that is, 

 the chromatin of the daughter nuclei is formed by the fusion of 

 the chromosomes of each group to form a skein at each pole, 

 which then becomes transformed into the karyomitomc charac- 

 teristic of the nucleus. At this time, constriction of the proto- 

 plasm of the cell takes place in the neighbourhood of tiie equator 

 of the spindle ; the spindle disappears, and linally each daughter 

 cell, with its full complement of chromosomes, becomes an entity. 



The nucleus is composed of material of fairly high " permea- 

 InHty " and, therefore, may be expected to travel towards the 

 equatorial axis. This is found to be so. In some cases the 

 nucleus is wholly, and in other instances it is only partially, drawn 

 into the field between the centrosomes. Differences, too, exist 

 in the relative development of asters and spindle which are 

 capable of explanation by analogy to the magnetic model. If, 

 in the experiment with iron filings, the field were surrounded by an 

 iron ring, the majority of the lines of force woidd pass round by 

 the ring. That is, the interpolar lines would be slight and the 

 extra-polar rays would be heavy. Similarly, we may correlate 

 a mitotic figure having good astral rays and a poor spindle with a 

 marked " permeability " of the surface of the cell. 



One would be entering the realms of pure hypothesis if physico- 

 chemical interpretations were attempted of the various stages of 

 karyokinesis. The constitution of protoplasm — vaguely stated 

 as an emulsion of various lipoids in a complex protein-water 

 emulsoid with various crystalloids in solution or adsorbed, presents 

 excellent opportunities for the theorist to draw parallels between 

 certain manifestations of force in living things and in dead matter. 

 The mechanisms underlying these processes are as yet unknown. 

 The processes themselves, like all other changes in matter, are 

 accompanied by changes in electrical potential. These changes 

 are measurable, and are not constant, but fluctuate (even reversing 

 in direction) at epochs coinciding with phases of development. 



Cause of cell division. We are now in a position to consider the 

 actual cause of cell division. To state xvhy the cell must divide — 

 to argue from a surface-volume ratio — is to presuppose a cell 

 consciousness or to postulate an external directing force — both 

 alternatives being outside the domain of physics. The use of the 

 final cause, or the argument that division is of obvious advantage 

 to the cell, sheds no light on the mechanism involved. Considera- 

 tion must be given to the forces at work in the cell. Further, 

 experiments such as the much quoted one of Brailsford Robertson. 



31—2 



