622 



THE FORMS OF TISSUES 



[CH. 



arrangement was that of our type 6 6 (Fig. 245), though the central 

 intermediate partition has been crowded out by partial coalescence. 

 When with increasing age the septa become more numerous, their 

 arrangement becomes exceedingly variable; for the simple reason 

 that, from the mathematical point of view, the number of possible 

 arrangements, of 10, 12 or more cellular partitions in triple contact, 

 tends to increase with great rapidity, and there is little to choose 

 among many of them in regard to symmetry and equihbrium. But 

 while, mathematically speaking, each particular case among the 



Fig. 271. Heterophyllia sp. After Martin Duncan. 



multitude of possible cases is an orderly and definite arrangement, 

 from the purely biological point of view on the other hand no law 

 or order is recognisable; and so McCoy described the genus as 

 being characterised by the possession of septa 'destitute of any 

 order of arrangement, but irregularly branching and coalescing in 

 their passage from the sohd external walls towards some indefinite 

 point near the centre where the few main lamellae irregularly 

 anastomose." 



In the two examples figured (Fig. 271 B, C), both comparatively 

 simple ones, it will be seen that, of the main chambers, one is in each 



