IV] AND STRUCTURE OF THE CELL 295 



and consisting of portions of "chromatin," aggregated together 

 within a more fluid drop. The fact has often been commented 

 upon that, in cells generally, there is no correlation of form 

 (though there apparently is of size) between the nucleus and the 

 "cytoplasm," or main body of the cell. So Whitman* remarks 

 that "except during the process of division the nucleus seldom 

 departs from it^ cypical spherical form. It divides and sub-divides, 

 ever returning to the same round or oval form. . . . How different 

 with the cell. It preserves the spherical form as rarely as the 

 nucleus departs from it. Variation in form marks the beginning 

 and the end of every important chapter in its history." On simple 

 dynamical grounds, the contrast is easily explained. So long as 

 the fluid substance of the nucleus is qualitatively different from, 

 and incapable of mixing with, the fluid or semi-fluid protoplasm 

 surrounding it, we shall expect it to be, as it almost always is, of 

 spherical form. For on the one hand, it has a surface of its own 

 whose surface-tension is presumably uniform, and on the other, it 

 is immersed in a medium which transmits on all sides a uniform 

 fluid or "hydrostatic" pressure f; thus the case of the spherical 

 nucleus is closely akin to that of the spherical yolk within the 

 bird's egg. Again, for a similar reason, the contractile vacuole of 

 a protozoon is spherical {. It is just a drop of fluid, bounded by a 



* Whitman, Journ. Morph. ii, p. 40, 1889. 



t "Souvent il n'y a qu'une separation physique entre le cytoplasme et le sue 

 hucleaire, comme entre deux liquides immiscibles, etc."; Alexeieff, 8ur la mitose 

 dite primitive, Arch. f. Protistenk. xxix, p. 357, 1913. 



X The appearance of " vacuolation " is a result of endosmosis, or the diffusion 

 of a less dense fluid into the denser plasma of the cell. But while water is probably 

 taken up at the surface of the cell by purely passive osmotic intake, a definite 

 "vacuole" appears at a place where osmotic work is being actively done. A higher 

 osmotic pressure than that of the external medium is maintained within the cell, 

 but as a "steady state" rather than a condition of equilibrium, in other words by 

 the continual expenditure of energy; and the difference of pressure is at best small. 

 The "contractile vacuole" bursts when it touches the surface of the cell, and 

 bursting may be delayed by manipulating the vacuole towards the interior. It 

 may sometimes burst towards the interior of the cell through inequalities in its 

 own surface-tension, and the collapsing vacuole is then apt to shew a star-shaped 

 figure. The cause of the higher osmotic pressure within the cell is a matter for 

 the colloid chemist, and cannot be discussed here. On the physiology of the 

 contractile vacuole, see {int. al.) H. Z. Gow, Arch. f. Protistenk. lxxxvii, pp. 185- 

 212, 1936; J. Spek, Einfluss der Salze auf die Plasmkolloide von Actinosphaerium, 

 Acta Zool. 1921; J. A, Kitching, Journ. Exp. Biology, xi, xiii, xv, 1934-38. 



