164 ON THE INTERNAL FORM AND [ch. 



though physical in the concrete, is in the abstract purely mathe- 

 matical, and in its very essence is neither more nor less than a 

 property of three-dimensional space. 



But as a matter of fact, in this instance, that is to say in 

 trying to explain the leading phenomena of the caryokinetic 

 division of the cell, we shall soon perceive that any explanation 

 which is based, like Rhumbler's, on mere mechanical traction, is 

 obviously inadequate, and we shall find ourselves limited to the 

 hypothesis of some polarised and polarising force, such as we deal 

 with, for instance, in the phenomena of magnetism or electricity. 



Let us speak first of the cell itself, as it appears in a state of 

 rest, and let us proceed afterwards to study the more active 

 phenomena which accompany its division. 



Our typical cell is a spherical body ; that is to say, the uniform 

 surface-tension at its boundary is balanced by the outward 

 resistance of uniform forces within. But at times the surface- 

 tension may be a fluctuating quantity, as when it produces the 

 rhythmical contractions or "Ransom's waves" on the surface of 

 a trout's egg; or again, while the egg is in contact with other 

 bodies, the surface-tension may be locally unequal and variable, 

 giving rise to an amoeboid figure, as in the egg of Hydra*. 



Within the ovum is a nucleus or germinal vesicle, also spherical, 



and consisting as a rule of portions of "chromatin," aggregated 



together within a more fluid drop. The fact has often been 



commented upon that, in cells generally, there is no correlation 



of form (though there apparently is of size) between the nucleus 



and the "cytoplasm," or main body of the cell. So Whitman "f 



remarks that "except during the process of division the nucleus 



seldom departs from its typical spherical form. It divides and 



sub-divides, ever returning to the same round or oval form.... 



How different with the cell. It preserves the spherical form as 



rarely as the nucleus departs from it. Variation in form marks 



the beginning and the end of every important chapter in its 



tension or attraction along the lines, and of repulsion transverse to the hnes, of tlie 

 diagram. 



* Cf. also the curious phenomenon in a dividing egg described as "spinning" 

 by Mrs G. F. Andrews, J. of Morph. xn, pp. 367-389, 1897. 



t Whitman, J. of Morph. n, p. 40, 1889. 



J 



