42 CELL-DIVISION AND GROWTH II. i 



further toward the end, it comes to another critical position 

 when it must lie across one corner, forming so the base of 

 a pyramid, or octant of a sphere. This position is defined by 



2 

 the equation r l = -- a, where r r is the radius of this sphere. 



3 



It is impossible, therefore, for a very flat cell, or short cylinder, to 

 be divided in conformity with the principle parallel to its longest 

 side, and yet this occurs, as, for instance, in cambium cells. 



It will also be noticed that this principle does not explain why 

 one particular direction is selected when many are apparently 

 equally possible. 



"We turn now to a consideration of the remaining factor which 

 assists in determining the shape of the cells and so the geo- 

 metrical pattern of segmentation ; this is the movement of the 

 cells upon one another. 



That such movement does occur we have already seen ; the 

 question which immediately suggests itself is whether in taking 

 up their new positions the cells obey the laws of capillarity as 

 enunciated for systems of fluid lamellae such as soap-bubbles by 

 Plateau in his principle of least surfaces. 



This principle, as we have seen, demands that the sum of 

 the external surfaces should be, under the conditions, a minimum, 

 or, expressed in physical rather than in geometrical language, 

 that the total surface energy should be minimal. In accordance 

 with this doctrine of minimal surface energy a drop of fluid 

 floating in a fluid medium assumes, as need hardly be said, the 

 form of a sphere. In a system of drops contact surfaces will be 

 formed between the drops, provided that each possesses a coating 

 film which has a positive energy with the media it separates ; 

 a film, that is, of such a nature that the total surface energy 

 would be diminished by apposition, without, however, involving 

 the disappearance of the separating film and fusion of the drops. 

 In other words, the film must be insoluble in both the external 

 and the internal media. A simple example of this is afforded 

 by the behaviour of the spheres of jelly covering the eggs of the 

 Frog, when taken from water and floated between chloroform 

 and benzole. Two or more such drops of jelly cohere by their 

 coating films, and form systems of lamellae the films, that is, 

 at the external surfaces and between the opposed surfaces of the 



