164 THE EVOLUTION OF CONTINUITY 



water-pressure resists. The density of the chloroform is, 

 however, such that a drop under water cannot assume a 

 spherical form. A drop of oil, on the other hand, suspended 

 in an emulsion, is spherical, its suspension and shape indi- 

 cating an even balance of pressure forces on all sides. 



Now a cell-mass, whose cells were all of the same density 

 and heavier than water, would under water-pressure develop 

 as a spread-out mat and not as a spherical mass. But if a 

 slight diminution in density went hand in hand with cell- 

 multiplication a spherical shape would almost certainly be 

 the result. And if there was basal attachment to the bottom, 

 the pull outwards and upwards due to diminishing density 

 would cause the formation of a hollow interior in the sphere, 

 in the presence of surrounding water-pressure. As cell- 

 multiplication proceeded, gradually progressing diminution 

 of density would, in presence of lateral water-pressure, cause 

 the sphere to elongate into a pear-shape, and finally into 

 the shape of a cylinder, and it is on these lines that it is 

 suggested that the zooid evolved. The budded Hydra 

 zooid, it may be noted, starts as a little hollow bud which 

 becomes pear-shaped, and then elongates into a cylindrical 

 form, a mouth opening distally from within outwards. 

 The last fact lends a little colour to the supposition that 

 the distal mouth of the new primitive zooid appeared when 

 the cells of the distal end of the cylinder were of such low 

 density that upward pull, assisted, it may be, by internal 

 pressure of fluid contents, caused a cell separation or break 

 which became the mouth. Pressure outside and inside the 

 cylinder would then be equal and the tubular form be main- 

 tained. The elongation of the tube would also tend to cease, 

 for a diminution of density sufficient to cause the outbreak 

 of a mouth would make for the discontinuity of further 

 cells produced. It may be that here we have the explana- 

 tion of the discrete, or laterally discontinuous tentacles round 

 the mouths of typical zooids, and these may have at first 

 appeared as final zooidal attempts at elongation ; diminished 

 density having made them stream upwards as separate 

 cell-strings, still attached, however, round the mouth of the 

 zooid. Thus, this would make gradual diminution of cell 

 density not only a chief factor in determining the shape of 

 the zooid, but also its proportionate length. It would 



