II] OF THE SIZE OF DROPS 59 



mass or volume during segmentation it is very slight indeed, 

 generally imperceptible, and wholly denied by some*. The growth 

 or development of the egg from a one-celled stage to stages of two 

 or many cells is thus a somewhat pecuhar kind of growth; it is 

 growth limited to change of form and increase of surface, unaccom- 

 panied by growth in volume or in mass. In the case of a soap-bubble, 

 by the way, if it divide into two bubbles the volume is actually 

 diminished, while the surface-area is greatly increased! ; the diminution 

 being due to a cause which we shall have to study later, namely to 

 the increased pressure due to the greater curvature of the smaller 

 bubbles. 



An immediate and remarkable result of the principles just 

 described is a tendency on the part of all cells, according to their 

 kind, to vary but little about a certain mean size, and to have in 

 fact certain absolute limitations of magnitude. The diameter of a 

 large parenchymatous cell is perhaps tenfold that of a httle one; 

 but the tallest phanerogams are ten thousand times the height of 

 the least. In short, Nature has her materials of predeterminate 

 dimensions, and keeps to the same bricks whether she build a great 

 house or a small. Even ordinary drops tend towards a certain 

 fixed size, which size is a function of the surface-tension, and may 

 be used (as Quincke used it) as a measure thereof. In a shower of 

 rain the principle is curiously illustrated, as Wilding Roller and 

 V. Bjerknes tell us. The drops are of graded sizes, each twice as big 

 as another, beginning with the minute and uniform droplets of an 

 impalpable mist. They rotate as they fall, and if two rotate in 

 contrary directions they draw together and presently coalesce; but 

 this only happens when two drops are faUing side by side, and since 

 the rate of fall depends on the size it always is a pair of coequal 

 drops which so meet, approach and join together. A supreme 

 instance of constancy or quasi-constancy of size, remote from but 

 yet analogous to the size-hmitation of a rain-drop or a cell, is the 

 fact that the stars of heaven (however else one diifereth from 

 another), and even the nebulae themselves, are all wellnigh co-equal 

 in mass. Gravity draws matter together, condensing it into a world 



* Though the entire egg is not increasing in mass, that is not to say that its living 

 protoplasm is not increasing all the while at the expense of the reserve material, 

 t Cf. P. G. Tait, Proc. E.S.E. v, 1866 and vi, 1868. 



