618 



THE FORMS OF TISSUES 



[CH. 



generally obey the rule of triple intersection, and accordingly 

 exemplify the partition-figures with which we have been dealing. 

 But whereas we have found the particular arrangement in which 

 one cell is in contact with all the rest to be unstable, according to 

 Roux's oil-drop experiments, and to be conspicuous by its absence 

 from our diagrams of segmenting eggs, here in Asterolampra, on the 

 other hand, it occurs frequently, and is indeed the commonest 

 arrangement (Fig, 266, B). In all probability, we are entitled to 

 consider this marked difference natural enough. For we may 

 suppose that in Asterolampra (unlike the 'case of the segmenting 

 egg) the tendency is to perfect radial sjnnmetry, all the spokes 

 emanating from a point in the centre: such a condition would be 



-IB c 



Fig. 266. (A) Asterolampra marylandica Ehr.; 

 (B, C) A. variabilis Grev. After Greville. 



eminently unstable, and would break down under the least asym- 

 metry. A very simple, perhaps the simplest case, would be that 

 one single spoke should differ slightly from the rest, and should so 

 tend to be drawn in amid the others, these latter remaining similar 

 and symmetrical among themselves. Such a configuration would 

 be vastly less unstable than the original one in which all the 

 boundaries meet in a point; and the fact that further progress is 

 not made towards other configurations' of still greater stability may 

 be sufficiently accounted for by viscosity, rapid solidification, or 

 other conditions of restraint. A perfectly stable condition would of 

 course be obtained if, as in the case of Roux's oil-drop (Fig. 257, 6), 

 one of the cellular spaces passed into the centre of the system, the 

 other partitions radiating outwards from its circular wall to the 

 periphery of the whole system. Precisely such a condition occurs 



