178 ON THE INTERNAL FORM AND [ch. 



the centrosonies the nucleus appears to act as a more or less perme- 

 able body, as a body more permeable than the surrounding medium, 

 that is to say the "cytoplasm" of the cell. It is accordingly 

 attracted by, and drawn into, the field of force, and tries, as it 

 were, to set itself between the poles and as far as possible from 

 both of them. In other words, the centrosome-foci will be 

 apparently drawn over its surface, until the nucleus as a whole 

 is involved within the field of force, which is visibly marked out 

 by the "spindle" (par. 3, Figs. 44, 45). 



If the field of force be electrical, or act in a fashion analogous 

 to an electrical field, the charged nucleus will have its surface- 

 tensions diminished * : with the double result that the inner 

 alveolar meshwork will be broken up (par. 1), and that the 

 spherical boundary of the whole nucleus will disappear (par. 2). 

 The break-up of the alveoh (by thinning and rupture of their 

 partition walls) leads to the formation of a net, and the further 

 break-up of the net may lead to the unravelling of a thread or 

 "spireme" (Figs. 43, 44). 



Here there comes into play a fundamental principle which, 

 in so far as we require to understand it, can be explained in simple 

 words. The effect (and we might even say the object) of drawing 

 the more permeable body in between the poles, is to obtain an 

 "easier path" by which the lines of force may travel; but it is 

 obvious that a longer route through the more permeable body 

 may at length be found less advantageous than a shorter route 

 through the less permeable medium. That is to say, the more 

 permeable body will only tend to be drawn in to the field of force 

 until a point is reached where (so to speak) the way round and 

 the way through are equally advantageous. We should accordingly 

 expect that (on our hypothesis) there would be found cases in 

 which the nucleus was wholly, and others in which it was only 

 partially, and in greater or less degree, drawn in to the field 

 between the centrosomes. This is precisely what is found to 

 occur in actual fact. Figs. 44 and 45 represent two so-called 

 "types," of a phase which follows that represented in Fig. 43. 

 According to the usual descriptions (and in particular to Professor 



* On the effect of electrical influences in altering the surface-tensions of the 

 colloid particles, see Bredig, Anorganische Fermente, pp. 15, 16, 1901. 



