488 THE FORMS OF TISSUES [ch. 



molecules is supposed to slip from one lozenge-configuration to an 

 opposite one, passing on the way through the simple cross or square 

 — a configuration of "higher energy" and less stable equilibrium*. 



The sohd geometry of this four-celled figure is not without 

 interest. If the two polar furrows (the one above and the other 

 below) run criss-cross, the whole is a more or less flattened and 

 distorted spherical tetrahedron. If they run parallel, then it is a 

 four-sided lozenge with two curved quadrilateral faces, and two 

 bilateral faces each bounded by two curved edges, Hke the "liths" 



Fig. 173. Examples of the "polar furrow". A, Pollen-grains (tetrads) of Neottia. 

 B, Egg of hookworm {Ankylostoma). C, First cells of a wasp's nest (Polistes). 

 (From Packard, after Saussure.) D, Four-celled stage of Volvox: from Janet. 

 E, Hair of Salvia, after Hanstein. 



of an orange I . In either case the lozenge-configuration is under 

 some restraint to keep its four cells in a plane; for a tetrahedral 

 pile, or pyramid, of four spheres would be the simplest arrangement 

 of all. 



The polar furrow and the partition of which it forms an edge are, like all 

 the edges and partitions in our associated cells, perfectly definite in dimensions 

 and position; and to draw them to scale, in projection, is a simple matter. 

 Taking the simplest case, when the radii of all four cells are equal to one another, 

 let c, c\ c" and c'" be the centres of the four cells, Fig. 174. The centres of 



* Cf. J. D. Bernal, Proc. R.S. (A), No. 914, p. 321, 1937. 



t The geometer seldom takes account of such two-sided surfaces or facets; 

 but in groups of cells or bubbles they are of common occurrence, and in the theory 

 of polyhedra they fit in without difficulty with the rest (cf. infra, p. 737). 



