232 THE FORMS OF CELLS [ch. 



of surface tension which we have now explained. The thread is 

 formed out of the fluid secretion of a gland, and issues from the 

 body as a semi-fluid cylinder, that is to say in the form of a surface 

 of equilibrium, the force of expulsion giving it its elongation and 

 that of surface tension giving it its circular section. It is prevented, 

 by almost immediate solidification on exposure to the air, from 

 breaking up into separate drops or spherules, as it would otherwise 

 tend to do as soon as the length of the cyhnder had passed its 

 limit of stabihty. But it is otherwise with the sticky secretion 

 which, coming from another gland, is simultaneously poured over 

 the issuing thread when it is to form the spiral portion of the 

 web. This latter secretion is more fluid than the first, and retains 

 its fluidity for a very much longer time, finally drying up after 

 several hours. By capillarity it "wets" the thread, spreading 

 itself over it in an even film, which film is now itself a cylinder. 

 But this liquid cylinder has its limit of stability when its length 

 equals its own circumference, and therefore just at the points so 

 defined it tends to disrupt into separate segments : or rather, in 

 the actual case, at points somewhat more distant, owing to the 

 imperfect fluidity of the viscous film, and still more to the frictional 

 drag upon it of the inner solid cylinder, or thread, with which it 

 is in contact. The cylinder disrupts in the usual manner, passing 

 first into the wavy outline of an unduloid, whose swollen portions 

 swell more and more till the contracted parts break asunder, and 

 we arrive at a series of spherical drops or beads, of equal size, 

 strung at equal intervals along the thread. If we try to spread 

 varnish over a thin stretched wire, we produce automatically the 

 same identical result * ; unless our varnish be such as to dry almost 

 instantaneously, it gathers into beads, and do what we can, we 

 fail to spread it smooth. It follows that, according to the viscidity 

 and drying power of the varnish, the process may stop or seem to 

 stop at any point short of the formation of the perfect spherules ; 

 it is quite possible, therefore, that as our final stage we may only 

 obtain half-formed beads, or the wavy outline of an unduloid. 

 The formation of the beads may be facilitated or hastened by 

 jerking the stretched thread, as the spider actually does: the 



* Felix Plateau recommends the use of a weighted thread, or plumb-line, 

 drawn up out of a jar of water or oil; Phil. Mag. xxxiv, p. 246, 1867. 



