390 THE FORMS OF CELLS [ch. 



edge becomes notched or sinuous, and the surface near by becomes 

 ribbed or fluted, owing to the internal flow being helped here and 

 hindered there by a viscous shear; and then all of a sudden the 

 uneven edge shoots out an array of tiny jets, which break up into 

 the countless droplets which constitute "spray" (Fig. 116). The 

 naturalist may be reminded also of the beautifully symmetrical 

 notching of the calycles of many hydroid zoophytes, which little 

 cups had begun their existence as Hquid or semi-hquid films before 

 they became stiff and rigid. The next phase of the splash (with 

 which we are less directly concerned) is that the crater subsides, 

 and where it stood a tall column rises up, which also tends, if it be 

 tall enough, to break up into drops. Lastly the column sinks down 

 in its turn, and a ripple runs out from where it stood. 



The edge of our little cup forms a hquid ring or annulus, com- 

 parable on the one hand to the edge of an advancing wave, and 

 on the other to a hquid thread or cylinder if only we conceive the 

 thread to be bent round into a ring; and accordingly, just as the 

 thread segments first into an unduloid and then into separate 

 spherical drops, so likewise will the edge of cup or annulus tend to 

 do. This phase of notching, or beading, of the edge of the splash 

 is beautifully seen in many of Worthington's experiments*, and still 

 more beautifully in recent work (Frontispiece f). In the second place 

 the fact that the crater rises up means that liquid is flowing in from 

 below; the segmentation of the rim means that channels of easier 

 flow are being created, along which the liquid is led or driven into 

 the protuberances; and these last are thereby exaggerated into the 

 jets or streams which become conspicuous at the edge of the crater. 

 In short any film or film-like fluid or semi-fluid cup will be unstable ; 

 its instability will tend to show itself in a fluting of the surface and 

 a notching of the edge; and just such a fluting and notching are 

 conspicuous features of many minute organic cup-like structures. 

 In the hydroids (Fig. 117), we see that these common features of the 



* Cf. A Study of Splashes, pp. 17, 77. The same phenomenon is often well seen 

 in the splash of an oar. It is beautifully and continuously evident when a strong 

 jet of water from a tap impinges on a curved surface and then shoots off again. 



t We owe this picture to the kindness of Mr Harold E. Edgerton, of the 

 Massachusetts Institute of Technology. It shews the splash caused by a drop 

 falling into a thin layer of milk; a second drop of milk is seen above, following 

 the first. The exposure-time was 1/50,000 of a second. 



