610 



THE FORMS OF TISSUES 



[CH. 



the aestivation of a flower, the spiral of a snail-shell, the scales on 

 a fir-cone, and many other common things. The theory of "spatial 

 complexes," as illustrated especially by knots, is a large part of 

 the subject. 



Topological analysis seems somewhat superfluous here; but it 

 may come into use some day to describe and classify such com- 

 pUcated, and diagnostic, patterns as are seen in the wings of a 

 butterfly or a fly. Let us look for a moment at how the topologist 

 might begin to study one of our groups of cells ; he would probably 



Fig. 260. 



call it an island divided into n counties, all maritime (i.e. none 

 encircled by the rest), and having inland none but three-way 

 junctions*. Here (in Fig. 260 a) is an island with nine counties; 

 and here (b) is a 9-gon, whose corners represent the same counties, and 

 the lines connecting these (whether sides or chords) represent the 

 contacts between. The polygon is now divided by six chords into 

 seven triangles. Three of these are peripheral, BCD, FGH, HJA ; 

 mark their vertices, C, G, J, each with the symbol 4, and obliterate 

 these three triangles (as in c). The remaining polygon has two 

 peripheral triangles BDE, EFH; obHterate these, after marking 



♦ This, like many another thing, comes from my good friend Dr G. T. Bennett. 



