VIIl] 



THE PARTITIONING OF SPACE 



393 



bent symmetrically downwards, so that the plane of the paper i? 

 transformed into a spheroidal surface, such as that of a hemisphere, 

 or that of a tall conical figure with curved sides ; let a membrane 

 be spread, umbrella-hke, between the outstretched skeletal rods, 

 and let its margin loop from rod to rod in curves which are possibly 

 catenaries, but are more probably portions of an "elastic curve," 

 and the outward resemblance to a Pluteus larva is now complete. 

 By various slight modifications, by altering the relative lengths 

 of the rods, by modifying their curvature or by replacing the curved 

 rod by a tangent to itself, we can ring the changes which lead us 

 from one known type of Pluteus to another. The case of the 

 Bipinnaria larvae of Echinids is certainly analogous, but it be- 

 comes very much more complicated ; we have to do with a more 



Fig. 177. Pluteus -larva of Ophiurid. 



complex partitioning of space, and I confess that I am not yet 

 able to represent the more compUcated forms in so simple a way. 



There are a few notable exceptions (besides the various un- 

 equally segmenting eggs) to the general rule that in cell-division 

 the mother-cell tends to divide into equal halves ; and one of these 

 exceptional cases is to be found in connection with the develop- 

 ment of "stomata" in the leaves of plants. The epidermal cells 

 by which the leaf is covered may be of various shapes ; sometimes, 

 as in a hyacinth, they are oblong, but more often they have an 

 irregular shape in which we can recognise, more or less clearly, 

 a distorted or imperfect hexagon. In the case of the oblong cells, 

 a transverse partition will be the least possible, whether the cell 

 be equally or unequally divided, unless (as we have already seen 



