392 THE FORMS OF CELLS [ch. 



evanescent rapidity in a more mobile liquid. Nor is there anything 

 pecuhar in the splash itself; it is simply a convenient method of 

 setting up certain motions or currents, and producing certain surface- 

 forms, in a Hquid medium — or even in such an imperfect fluid as a bed 

 of sand. Accordingly, we have a large range of possible conditions 

 under which the organism might conceivably display configurations 

 analogous to, or identical with, those which Mr Worthington has 

 shewn us how to exhibit by one particular experimental method. 



To one who has watched the potter at his wheel, it is plain that 

 the potter's thumb, like the glass-blower's blast of air, depends for 

 its efficacy upon the physical properties of the clay or "shp" it 

 works on, which for the time being is essentially a fluid. The cup 

 and the saucer, like the tube and the bulb, display (in their simple 

 and primitive forms) beautiful surfaces of equilibrium as manifested 

 under certain hmiting conditions. They are neither more nor less 

 than glorified "splashes," formed slowly, under conditions of 

 restraint which enhance or reveal their mathematical symmetry. 

 We have seen, and we shall see again before we are done, that the 

 art of the glass-blower is full of lessons for the naturahst as also 

 for the physicist: illustrating as it does the development of a host 

 of mathematical configurations and organic conformations which 

 depend essentially on the establishment of a constant and uniform 

 pressure within a closed elastic shell or fluid envelope or bubble. 

 In hke manner the potter's art illustrates the somewhat obscurer 

 and more complex problems (scarcely less frequent in biology) of a 

 figure of equilibrium which is an open surface of revolution. The 

 two series of problems are closely akin; for the glass-blower can 

 make most things which the potter makes, by cutting off portions 

 of his hollow ware; besides, when this fails and the glass-blower, 

 ceasing to blow, begins to use his rod to trim the sides or turn the 

 edges of wineglass or of beaker, he is merely borrowing a trick from 

 the still older craft of the potter. 



It would seem venturesome to extend our comparison with these 

 liquid surface-tension phenomena from the cup or calycleof the 

 hydrozoon to the little hydroid polyp within: and yet there is 

 something to be learned by such a comparison. The cylindrical 

 body of the tiny polyp, the jet-hke row of tentacles, the beaded 



