440 THE FORMS OF CELLS [ch. 



In all these cases, just as in the case of the blood-corpuscle, the 

 structure alters, and finally loses, its characteristic form when the 

 constitution of the surrounding medium is changed*. 



Here again, as in the blood-cOrpuscle, we have to do with the 

 important force of osmosis, manifested under conditions similar 

 to those of Pfeffer's classical experiments on the plant-cell f. The 

 surface of the cell acts as a semi-permeable membrane, permitting 

 the passage of certain dissolved substances (or their ions), and 

 including or excluding others: and thus rendering manifest and 

 measurable the existence of a definite "osmotic pressure." Again, 

 in the hen's egg a delicate yolk-membrane separates the yolk from 

 the white. The morphologist looks on it but as the cell-wall of 

 a vast yolk-laden germ-cell; the physiologist sees in it a semi- 

 permeable membrane, the seat of many complex activities. The 

 end and upshot of these last is that a steady difference of osmotic 

 pressure, the equivalent of some two atmospheres, is maintained 

 between yolk and white ; and yet there is no current flowing through. 

 Somewhere or other in the system there is a constant metabolic 

 flux, a continuous liberation of energy, a continual doing of work, 

 all leading to the maintenance of a steady dynamical state, which 

 is not " equilibrium {." 



In the case of the sperm-cells of Inachus, certain quantitative 

 experiments have been performed. The sperm-cell exhibits its 

 characteristic conformation while lying in the serous fluid of the 

 animal's body, in ordinary sea-water, or in a 5 per cent, solution 

 of potassium nitrate, these three fluids being all "isotonic" with 

 one another. As we alter the concentration of potassium nitrate, 

 the cell assumes certain definite forms corresponding to definite 

 concentrations of the salt; and, as a further and final proof that 

 the phenomenon is entirely physical, it is found that other salts 

 produce an identical effect when their concentration is proportionate 

 to their molecular weight, and whatever identical effect is produced 



* Cf. N. K. KoltzofF, Studien iiber die Gestalt der Zelle, Arch. f. mikrosk. Anat. 

 Lxvii, pp. 365-572, 1905; Biol. Centralhl. xxm, pp. 680-696, 1903; xxvi, pp. 854- 

 863, 1906; XLvm, pp. 345-369, 1928; Arch. f. Zellforschung, u, pp. 1-65, 1908; 

 vn, pp. 344^23, 1911; Anat. Anzeiger, xli, pp. 183-206, 1912. 



t W Pfeffer, Osmotische Untersuchungen, Leipzig, 1877. 



X Cf. J. Straub, Der Unterschied in osmotischer Konzentration zwischen Eigelb 

 und Eiklar, Rer. Trav. Chim. du Pays-Bas, XLvni, p. 49, 1929. 



