240 THE FORMS OF CELLS [ch. 



stability of our cylinder. It is for this reason that the mercury 

 in our thermometer tubes does not as a rule separate into drops, 

 though it occasionally does so, much to our inconvenience. And 

 again it is for this reason that the protoplasm in a long and growing 

 tubular or cylindrical cell does not necessarily divide into separate 

 cells and internodes, until the length of these far exceeds the 

 theoretic limits. Of course however and whenever it does so, we 

 must, without ever excluding the agency of surface tension, 

 remember that there may be other forces affecting the latter, and 

 accelerating or retarding that manifestation of surface tension by 

 which the cell is actually rounded off and divided. 



In most liquids. Plateau asserts that, on the average, the 

 influence of viscosity is such as to cause the cylinder to segment 

 when its length is about four times, or at most from four to six 

 times that of its diameter : instead of a fraction over three times 

 as, in a perfect fluid, theory would demand. If we take it at 

 four times, it may then be shewn that the resulting spheres would 

 have a diameter of about 1-8 times, and their distance apart would 

 be equal to about 2-2 times the diameter of the original cylinder. 

 The calculation is not difficult which would shew how these 

 numbers are altered in the case of a cylinder formed around a solid 

 core, as in the case of the spider's web. Plateau has also made 

 the interesting observation that the time taken in the process of 

 division of the cylinder is directly proportional to the diameter 

 of the cylinder, while varying considerably with the nature of the 

 liquid. This question, of the time occupied in the division of a 

 cell or filament, in relation to the dimensions of the latter, has not 

 so far as I know been enquired into by biologists. 



From the simple fact that the sphere is of all surfaces that 

 whose surface-area for a given volume is an absolute minimum, 

 we have already seen it to be plain that it is the one and only 

 figure of equilibrium which will be assumed under surface-tension 

 by a drop or vesicle, when no other disturbing factors are present. 

 One of the most important of these disturbing factors will be 

 introduced, in tjie form of complicated tensions and pressures, 

 when one drop is in contact with another drop and when a system 

 of intermediate films or partition walls is developed between them. 



