270 THE FORMS OF CELLS [ch. 



(which we find in oysters), a very similar undulating membrane 

 exists, but it is coiled in a regular spiral round the body of the cell. 

 It forms a "screw-surface," or helicoid, and, though we might 

 think that nothing could well be more curved, yet its mathematical 

 properties are such that it constitutes a "ruled surface" whose 

 "mean curvature" is everywhere nil; and this property (as we 

 have seen) it shares with the plane, and with the plane alone. 

 Precisely such a surface, and of exquisite beauty, may be 

 produced by bending a wire upon itself so that part forms an 

 axial rod and part a spiral wrapping round the axis, and then 

 dipping the whole into a soapy solution. 



These undulating and helicoid surfaces are exactly reproduced 

 among certain forms of spermatozoa. The tail of a spermatozoon 

 consists normally of an axis surrounded by clearer and more fluid 

 protoplasm, and the axis sometimes splits up into two or more 

 slender filaments. To surface tension operating between these 

 and the surface of the fluid protoplasm (just as in the case of the 

 fiagellum of the Trypanosome), I ascribe the formation of the 

 undulating membrane which we find, for instance, in the spermato- 

 zoa of the newt or salamander; and of the helicoid membrane, 

 wrapped in a far closer and more beautiful spiral than that which 

 we saw in Spirochaeta, which is characteristic of the spermatozoa 

 of many birds. 



Before we pass from the subject of the conformation of the 

 solitary cell we must take some account of certain other exceptional 

 forms, less easy of explanation, and still less perfectly understood. 

 Such is the case, for instance, with the red blood-corpuscles of man 

 and other vertebrates ; and among the sperm-cells of the decapod 

 Crustacea we find forms still more aberrant and not less perplexing. 

 These are among the comparatively few cells or cell-like structures 

 whose form seenis to be incapable of explanation by theories of 

 surface-tension. 



In all the mammalia (save a very few) the red blood-corpuscles 

 are flattened circular discs, dimpled in upon their two opposite 

 sides. This configuration closely resembles that of an india- 

 rubber ball when we pinch it tightly between finger and thumb ; 

 and we may also compare it with that experiment of Plateau's 



