VIII] OF THE VENATION OF WINGS 613 



only by the condition that the walls impinge on one another three 

 by three; and this being so, the assemblage includes a number of 

 small intermediate partitions analogous to the "polar furrows" of 

 embryology. We have seen how complex such configurations become 

 as the cells increase in number; and another source of complexity 

 comes in when the veins are of varying thickness and unequal tension, 

 and hence meet one another at varying angles. 



The entomologist is much concerned with the number and 

 arrangement of these veins. In Fig. 263 we shew three forewings 

 of a certain stonefiy, which serve first to shew how constantly the 

 veins meet in three-way junctions; and then we notice how the 



Fig. 262. Forewing of cicada. 



three wings are not exactly alike, for all their close resemblance, 

 because in two of them there is one cell less than in the third, a being 

 confluent with h in one of these cases and with c in the gther*. In 

 other words, the veinlet ah has gone amissing in the one, and ac in 

 the other, and in each case the remaining veinlet has sprung into a 

 position of equihbrium. This is one of more than two hundred 

 variations which have been recorded in the wing-veins of this one 

 insect; it might seem superfluous to look for more. 



The lower algae shew us many beautiful patterns or collocations 

 of cells, sometimes very compUcated, as in Volvox or Hydrodictyon. 

 A simpler case is that of Gonium. Of its sixteen cells four commonly 

 form a square, being so thrust apart out of closer packing by accu- 

 mulated intercellular substance; the other twelve are grouped 

 around these four, and obey the rule that three cells and no more 

 meet at each node or point of contact (Fig. 264). The twelve cells 



* From Arthur Willey, Graded mutations in wings of a stonefly {Allocapnia 

 pygmaea Burm.), Nature, July 17, 1937. 



