310 ON THE INTERNAL FORM [ch. 



the general principles of physical science. But when we speak of 

 some particular force or mode of action, using it as an illustrative 

 hypothesis, we stop far short of the implication that this or that 

 force is necessarily the very one which is actually at work within 

 the living cell; and certainly we need not attempt the formidable 

 task of trying to reconcile, or to choose between, the various 

 hypotheses which have already been enunciated, or the several 

 assumptions on which they depend. 



Many other things happen within the cell, especiall)^ in the germ- 

 cell both before and after fertilisation. They also have a physical 

 element, or a mechanical aspect, like the phenomena of cell- 

 division which we are speaking of; but the narrow bounds to which 

 we are keeping hold difficulties enough*. 



Any region of space within which action is manifested is a field 

 of force; and a simple example is a bipolar field, in which the 

 action is symmetrical with reference to the fine joining two points, 

 or poles, and with reference also to the "equatorial" plane equi- 

 distant from both. We have such a field of force in the neigh- 

 bourhood of the centrosome of the ripe cell or ovum, when it is 

 about to divide; and by the time the centrosome has divided, the 

 field is definitely a bipolar one. 



The quality of a medium filling the field of force may be uniform, 

 or it may vary from point to point. In particular, it may depend 

 upon the magnitude of the field; and the quality of one medium 

 may differ from that of another. Such variation of quality, within 

 one medium, or from one medium to another, is capable of diagram- 

 matic representation by a variation of the direction or the strength 

 of the field (other conditions being the same) from the state 

 manifested in some uniform medium taken as a standard. The 

 medium is said to be permeable to the force, in greater or less degree 

 than the standard medium, according as the variation of the density 

 of the lines of force from the standard case, under otherwise identical 

 conditions, is in excess or defect. A body placed in the medium will 

 tend to move towards regions of greater or less force according as its 



* Cf. C. D. Darlington, JieretU Advances in Cytology, 1932, and other well-known 

 works. 



