II] THE SIZE OF CELLS 35 



the conditions which favour equihbrium will be greatly altered by 

 the changed ratio of external surface to mass which a change of 

 magnitude, unaccompanied by change of form, produces in the cell. 

 Li short, however it may be brought aboi^t, the phenomenon of 

 division of the cell will be precisely what is required to keep 

 approximately constant the ratio between surface and mass, and 

 to restore the balance between the surface-energy and the other 

 energies of the system. When a germ-cell, for instance, divides 

 or "'segments" into two, it does not increase in mass; at least if 

 there be some shght alleged tendency for the egg to increase in 

 mass or volume during segmentation, it is very slight indeed, 

 generally imperceptible, and wholly denied by some*. The 

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

 stages of two or many cells, is thus a somewhat peculiar kind 

 of growth; it is growth which is limited to increase of surface, 

 unaccompanied 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 f, while the surface-area 

 is greatly increased. This is 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. 



Sachs J pointed out, in 1895, that there is a tendency for each 

 nucleus to be only able to gather around itself a certain definite 

 amount of protoplasm. Driesch§, a little later, found that, by 

 artificial subdivision of the egg, it was possible to rear dwarf 

 sea-urchin larvae, one-half, one-quarter, or even one-eighth of their 



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

 living protoplasm is not increasing all the whUe at the expense of tbe reserve 

 material. 



t Of. Tait, Proc. R.S.E. v, 1866, and vi, 1868. 



X Physiolog. Notizen (9), p. 425, 1895. Cf. Strasbiirger, Ueber die Wirkungs- 

 sphare der Kerne und die Zellgrosse, Histolog. Beitr. (5), pp. 95-129, 1893; 

 J. J. Gerassimow, Ueber die Grosse des Zellkernes, Beih. Bot. Centralbl. xvm, 

 1905 ; also G. Levi and T. Terni, Le variazioni dell' indice plasmatico-nucleare 

 durante 1' intercinesi. Arch. Ital. di Anat. x, p. 545, 1911. 



§ Arch. f. Entw. Mech. iv, 1898, pp. 75, 247. 



3—2 



