564 THE FORMS OF TISSUES [ch. 



of viscose, or artificial silk, tend likewise to have a ridged or fluted 

 structure, and gain in lustre thereby. The subject is new, and 

 hardly ripe for full discussion ; but it holds out promise (as it seems 

 to me) of many biological lessons and illustrations. 



We glanced in passing at such "shrinkage-patterns" as are found, 

 for instance, on the little shells of Lagena, or on those other hanging 

 drops which constitute Emil Hatschek's artificial medusae; it 

 is no small subject. A stretched elastic membrane, circular or 

 spherical, remains spherical or circular when we let its tension 

 relax; but if, to begin with, we coat the rubber with a pliant but 

 non-elastic material such as wax, the waxen layer, failing to con- 



Fig. 218. Amphicoelous vertebrae of a shark. 



tract, is thrown into more or less characteristic folds. In a dried 

 pea the seed has shrunken through loss of moisture, and the loose 

 outer coat wrinkles up*. The pretty pattern of a poppy-seed arises 

 in the same way; but so do the wrinkles on an old man's withered 

 skin. When our experimental elastic with its non-contractile coat 

 is suffered to contract, the first sign of the coat's inability to keep 

 pace is the appearance of little domes, or hummocks, or bhsters; 

 and soon from each of these there run out folds, which tend to fork, 

 and the angles between the three branches tend to equalise. They 

 tend, in simple and symmetrical cases, to form a pattern of hexagons, 

 with occasional pentagons or quadrilaterals between ; but where the 

 surface is larger and the coat more flexible the folds form an irregular 

 network, still with the various anticlines mostly meeting in three- 



* The difference between a smooth and a wrinkled pea, familiar to Mendelians, 

 merely depends, somehow, on amount and rate of shrinkage. 



