168 ON THE INTERNAL FORM AND [ch. 



plate of glass, and let fall into the middle of it a drop of indian 

 ink, or of blood, we shall find the coloured particles travelUng 

 outwards from the central "pole of concentration " along the lines 

 of diffusive force, and so mapping out for us a "monopolar field" 

 of diffusion : and if we set two such drops side by side, their 

 lines of diffusion will oppose, and repel, one another. Or, instead 

 of the uniform layer of salt solution, we may place at a little 

 distance from one another a grain of salt and a drop of blood, 

 representing two opposite poles : and so obtain a picture of a 

 "bipolar field" of diffusion. In either case, we obtain results 

 closely analogous to the "morphological," but really dynamical, 

 polarity of the organic cell. But in all probabiUty, the dynamical 

 polarity, or asymmetry of the cell is a very complicated phenome- 

 non : for the obvious reason that, in any system, one asymmetry 

 ^^'i^ tend to beget another. A chemical asymmetry will induce an 

 inequahty of surface-tension, which will lead directly to a modifi- 

 cation of form ; the chemical asymmetry may in turn be due to a 

 process of electrolysis in a polarised electrical field; and again 

 the chemical heterogeneity may be intensified into a chemical 

 "polarity," by the tendency of certain substances to seek a locus 

 of greater or less surface-energy. We need not attempt to 

 grapple with a subject so complicated, and leading to so many 

 problems which lie beyond the sphere of interest of the morph- 

 ologist. But yet the morphologist, in his study of the cell, 

 cannot quite evade these important issues ; and we shall return 

 to them again when we have dealt somewhat with the form of 

 the cell, and have taken account of some of the simpler pheno- 

 mena of surface-tension. 



We are now ready, and in some measure prepared, to study 

 the numerous and complex phenomena which usually accompany 

 the division of the cell, for instance of the fertilised egg. 



Division of the cell is essentially accompanied, and preceded, 

 by a change from radial or monopolar to a definitely bipolar 

 polarity. 



In the hitherto quiescent, or apparently quiescent cell, we per- 

 ceive certain movements, which correspond precisely to what must 

 accompany and result from a "polarisation" of forces within the 



