V] OF THE SUN-ANIMALCULES 265 



of spheres. Outwards through the protoplasm, and stretching far 

 beyond the spherical surface of the cell, there run stiff linear 

 threads of modified or differentiated protoplasm, replaced or 

 reinforced in some cases by delicate sihceous needles. In either 

 case we know little or nothing about the forces which lead to their 

 production, and we do not hide our ignorance when we ascribe 

 their development to a "radial polarisation" of the cell. In the 

 case of the protoplasmic filament, we may (if we seek for a 

 hypothesis), suppose that it is somehow comparable to a viscid 

 stream, or "liquid vein," thrust or squirted out from the body of 

 the cell. But when it is once formed, this long and comparatively 

 rigid filament is separated by a distinct surface from the neigh- 

 bouring protoplasm, that is to say from the more fluid surface- 

 protoplasm of the cell ; and the latter begins to creep up the 

 filament, just as water would creep up the interior of a glass tube, 

 or the sides of a glass rod immersed in the liquid. It is the simple 

 case of a balance between three separate tensions : ( 1 ) that between 

 the filament and the adjacent protoplasm, (2) that between the 

 filament and the adjacent water, and (3) that between the water 

 and the protoplasm. Calling these tensions respectively Tf^.,, Tf^, 

 and T^,p, equilibrium will be attained when the angle of contact 

 between the fluid protoplasm and the filament is such that 



T — T 



cos a = ^^-, -^ . It is evident in this case that the angle is 



T 



fp 



a very small one. The precise form of the curve is somewhat 

 different from that which, under ordinary circumstances, is assumed 

 by a liquid which creeps up a solid surface, as water in contact 

 with air creeps up a surface of glass ; the difference being due to 

 the fact that here, owing to the density of the protoplasm being 

 practically identical with that of the surrounding medium, the 

 whole system is practically immune from gravity. Under normal 

 circumstances the curve is part of the "elastic curve" by which 

 that surface of revolution is generated which we have called, 

 after Plateau, the nodoid ; but in the present case it is apparently 

 a catenary. Whatever curve it be, it obviously forms a surface 

 of revolution around the filament. 



Since the attraction exercised by this surface tension is 

 symmetrical around the filament, the latter will be pulled equally 



