590 



THE FORMS OF TISSUES 



[CH. 



The case here illustrated is of no small importance. For it shews 

 us that a uniform and symmetrical growth of the organism (sym- 

 metrical, that is to say, under the limitations of a plane surface, 

 or plane section) by no means involves a uniform or symmetrical 

 growth of the individual cells, but may under certain conditions 

 actually lead to inequahty among these; and this phenomenon 

 (or to be quite candid, this hypothesis, which is due to Berthold) 

 is independent of any change or variation in surface tensions, 

 and is essentially different from that unequal segmentation (studied 

 by Balfour) to which we. have referred on p. 568. 



After this fashion we might go on to consider the manner, and 

 the order of succession, in which subsequent cell-divisions should 

 tend to take place, as governed by the principle of minimal areas. 



I 



Fig. 238. 



The calculations would grow more difficult, and the results •got 

 by simple methods would grow less and less exact; at the same 

 time some of the results would be of great interest, and well worth 

 our while to obtain. For instance, the precise manner in which our 

 triangular cell, T, would next divide would be interesting to know, 

 and a general solution of this problem is certainly troublesome to 

 calculate. But in this particular case we see that the width of the 

 triangular cell near P (Fig. 238) is so obviously less than that near 

 either of the other two angles, that a circular arc cutting off that 

 angle is bound to be the shortest possible bisecting line; and that, 

 in short, our triangular cell will tend to subdivide, just hke the 

 original quadrant, into a triangular and a quadrilateral portion. 



But the case will be different next time, because in this new 

 triangle,' PRQ, the least width is near the innermost angle, that 

 at Q\ and the bisecting circular arc will therefore be opposite to Q, 

 or (approximately) parallel to PR. The importance of this fact is 

 at once evident; for it means to say that there comes a time 



