74 Geometrical relation of Nuclei 



But if the tissues are only imperfectly compressible there will 

 then be a counteracting force exerted on Y by its neighbours at 

 the points of contact, tending to prevent the movement of Y 

 towards D. The strength of this counteracting force will depend 

 inter alia on the position of these points of contact. The nearer 

 they are to the point D, the greater will be their influence, the 

 further removed from D, the less will be their influence and the 

 less will be the tendency to prevent the movement of Y towards 

 B. Now clearly with a sphere (or ring of balls as in the diagram) 

 any enlargement of the sphere (or ring) will move the points of 

 contact further from D and diminish the counteracting force due 

 to physical pressure and will therefore favour the movement of 

 Y towards D. Thus the invagination of the lower pole of the 

 diagram depends upon (1) the presence of an attractive force acting 

 between sphere and sphere, (2) the difference, relative to the centre 

 of the mass, of the position of the centre from which the force acts 

 in the upper and lower parts of the ring, (3) the relative strengths 

 of the supposed inter-spheric attractions and the counteracting 

 physical pressure at the points of contact of sphere and 

 sphere. 



Let us see to what extent it is possible to find these conditions 

 in the developing blastula and gastrula in a typical case of invagina- 

 tion such as we get during the gastrulation of Amphioxus. In the 

 first place we must regard the phenomenon of intercellular attrac- 

 tion as an established fact, and believe that an attraction exists 

 between cell and cell as an attribute of a living cell, as much as 

 gravity is an attribute of all matter. 



Like gravity we must consider that it can be described as 

 acting from a centre. Just as there is a centre of gravity for 

 every mass, and just as the centre of gravity may be not the 

 actual centre of the mass, so there must always be an attraction 

 centre in a cell, and this attraction centre need not correspond to 

 the actual centre of the cell mass, nor to its centre of gravity. 

 Thus in a bird's ovum for instance, which we may consider to be 

 a sphere, the centre of gravity is certainly below the actual centre 

 of the sphere (i.e., nearer the "vegetative" pole) while its attraction 

 centre will probably be very much above the actual centre of the 

 sphere. 



