592 



THE FORMS OF TISSUES 



[CH. 



scarcely anything like the normal look of an aggregate of hving 

 cells. But let us go a little further, still hmiting ourselves to the 

 consideration of the eight-celled stage. Wherever one of our 

 radiating partitions meets the peripheral wall, there will (as we 

 know) be a mutual tension between the three convergent films, 

 which will tend to set their edges at equal angles to one another, 

 angles that is to say of 120°. In consequence of this, the outer wall 

 of each individual cell will (in this surface view of our disc) be an 

 arc of a circle of which we can determine the centre by the method 

 used on p. 485; and, furthermore, the narrower cells, that is to say 

 the quadrilaterals, will have this outer border somewhat more 



Fig. 239. Diagram of flattened or discoid cell dividing into octants : to shew 

 gradual tendency towards a position of equilibrium. 



curved than their broader neighbours. We arrive, then, at the 

 condition shewn in Fig. 239 b. Within the cell, also, wherever 

 wall meets wall, the angle of contact must tend, in every case, to 

 be an angle of 120°; in no case may more than three films (as seen 

 in section) meet in a point (c) ; and this condition, of the partitions 

 meeting three by three and at co-equal angles, will involve the 

 curvature of some, if not all, of the partitions (d) which to begin 

 with we treated as plane. To solve this problem in a general way 

 is no easy matter; but it is a problem which Nature solves in 

 every case where, as in the case we are considering, eight bubbles 

 or eight cells meet together in a plane or curved surface. An 

 approximate solution has been given in d; and it will at once be 

 recognised that this figure has vastly more resemblance to an 



