416 THE FORMS OF CELLS [ch. 



its own weight, aided by the viscosity of the material. Indeed the 

 various hanging drops, such as Mr C. R. DarUng shews us, are the 

 most beautiful and perfect unduloids, with spherical ends, that it 

 is possible to conceive. A suitable hquid, a httle denser than water 

 and incapable of mixing with it (such as ethyl benzoati), is poured 

 on a surface of water. It spreads over the surface and gradually 

 forms a hanging drop, approximately hemispherical; but as more 

 liquid is added the drop sinks or rather stretches downwards, still 

 adhering to the surface fihn; and the balance of forces between 

 gravity and surface-tension results in the unduloid contour, as the 

 increasing weight of the drop tends to stretch it out and finally 

 Sreak it in two. At the moment of rupture, by the way, a tiny 



Fig. 132. Large "hanging drops" ot oil. After Darling. 



droplet js formed in the attenuated neck, such as we described in 

 the normal division of a cyhndrical thread. 



The thin, fusiform, pointed, non-globular Lagenas are less easily 

 explained. Surface-tension, which tends to keep the drop spherical, 

 is overmastered here, and the elongate shape suggests the viscous 

 drag of a shearing fluid*. 



To pass to a more highly organised class of animals, v^e find the unduloid 

 beautifully exemplified in the little flask-shaped shells of certain Pteropod 

 mollusca, e.g. Cuvierina-\. Here again the symmetry of the figure would 

 at once lead us to suspect that the creature lived in a position of symmetry 

 to the surrounding forces, as for instance if it floated in the ocean in an 

 erect position, that is to say with its long axis coincident with the direction 

 of gravity; and this we know to be actually the mode of life of the little 

 Pteropod. 



* Cf. G. I. Taylor, The formation of emulsions in definable fields of flow, Proc. R.S. 

 (A), No. 858, p. 501, 1934. 

 t Cf. Boas, Spolia Atlantica, 1886, pi. 6. 



