VIII] THE PARTITIONING OF SPACE 387 



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. 171, B). In all proba- 

 bility, 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 

 symmetry, all the spokes emanating from a point in the centre: 

 such a condition would be eminently unstable, and would break 

 down under the least asymmetry. 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. 170, 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 among our diatoms; 

 but when it does so, it is looked 

 upon as the mark and characterisa- 

 tion of the allied genus Arachnoid- 

 iscus. 



In a diagrammatic section of 

 an Alcyonarian polype (Fig. 172), 

 we have eight chambers set, sym- 

 metrically, about a ninth, which 

 constitutes the "stomach." In this 



arrangement there is no difficulty, 



... Fig. 172. Section of Alcyonarian 



tor it IS obvious that, throughout polype. 



the system, three boundaries meet 



(in plane section) in a point. In many corals we have as 



* The same is true of the insect's wing; but in this case I do not hazard a 

 conjectural explanation. 



25—2 



