436 THE FORMS OF CELLS [ch. 



in another liquid. Some other force or forces must be at work to 

 conform it, and the simple effect of mechanical pressure is excluded, 

 because the corpuscle exhibits its characteristic shape while floating 

 freely in the blood. It has been suggested that the corpuscle is 

 perhaps comparable to a sohd of revolution described about one of 

 Cay ley's equipotential curves*, such as we have spoken of briefly 

 on p. 318. Were the corpuscle a sphere, or a thin plate, a gas 

 diffusing inwards would reach all parts equally soon ; but the surface 



would be small in the one case and 



-^_C ^ ^ the volume in the other. In so 



far as the corpuscle resembles or 

 approaches the equipotential form, 

 we might look on it as a com- 

 Fiff 14.^ ^" ^ promise; but however advan- 

 tageous such a shape might be, and 

 however interesting physiologically, we should be as far as ever 

 from understanding how it was produced. In all other vertebrates, 

 from fishes to birds, sluggish or active, warm-blooded or cold, the 

 blood-corpuscles have the simpler form of a flattened oval disc, 

 with somewhat sharp edges and eUipsoidal surfaces, and this again 

 is manifestly not a surface of fluid equilibrium. But there is 

 nothing to choose between the one type and the other in the way 

 of physiological efficiency, nor any apparent need for a refinement 

 of adaptive form in either of them. 



Two facts are noteworthy in connection with the form of the 

 mammalian blood-corpuscle. In the first place its form is only 

 maintained, that is to say it is only in equilibrium, in specific relation 

 to the medium in which it floats. If we add water to the blood, 

 the corpuscle becomes a spherical drop, a true surface of minimal 

 area and stable equiUbrium; if, conversely, we add a little salt, or 

 a drop of glycerine, the corpuscle shrinks, and its surface becomes 

 puckered and uneven. So far, it merely obeys the laws of diffusion; 

 but the phenomenon is more complex than this|. For the spherical 

 form is assumed just as well in various isotonic solutions, leaving 



♦ Cf. H. Hartridge, Journ. Physiol, lii, p. Ixxxi, 1919-20; Eric Ponder, Journ. 

 Gen. Physiol, ix, pp. 197-204, 625-629, 1925-26. 



I Cf. A. Gough, On the assumption of a spherical form by human blood- 

 corpuscles, Biochem. Journ. xvni, p. 202, 1924. 



