I CYTOPLASM 191 



On the other hand, fertilization of the sea-urchin egg is followed by 

 a solidification of the fluid endoplasm into a gel (Mirsky, 1956). 



f . Separation of the Cytoplasm into Different Phases 



As long as there exists a certain equilibrium between the cyto- 

 plasmic proteins on the one hand and the amount of lipids and water 

 on the other, the cytoplasm remains microscopically homogeneous, 

 hyaline, as clear as water and optically empty. In the physico-chemical 

 sense as well, the system is a homogeneous pseudophase (p. 69) 

 without inner surfaces. This system is bound to separate into phases 

 if one of the three components, protein, Hpid or solvent, increases in 

 quantity to such an extent that the state of mutual equilibrium can 

 no longer be maintained, and similar molecules cluster together and 

 are separated from the rest of the cytoplasm by a phase boundary. 



Formation of vacuoles. Guilliermond (1933) attributes the origin of 

 vacuoles to the formation of hydrophilic colloids in the cytoplasm. 

 These colloids attract water, are hydrated and thus cause a separation. 

 It is quite possible that the vacuoles are formed in this manner. Besides 

 colloids, salts, which accumulate in the cell, may initiate the accumu- 

 lation of water in some of the meshes of the submicroscopic frame- 

 work. Then, according to the laws of surface tension, the aqueous 

 phase becomes spherical in shape and pushes the framework aside.^ It 

 may therefore be assumed that the framework of the cytoplasm has 

 a higher density in the neighbourhood of a vacuole. Thereupon lipids 

 are accumulated in the boundary layer (cf. Fig. 115, p. 199). 



The colloid content of the vacuolar liquid can be demonstrated, or 

 at least shown to be probable, in several ways. The viscosity, for 

 instance, is about twice that of water (Weber, 1921; Pekarek, 1933) 

 or of aqueous solutions with the same salt content as the vacuoles 

 (cf. Table XXII, p. 169). The large terminal vesicle of Closterium algae, 

 in which the sedimentation of g^'psum crystals can be measured accu- 

 rately (Frey, 1926 c), is particularly suitable for the application of the 

 falling particle method. From Stokes' law one derives a relative vis- 

 cosity of about 2.5 for the cell sap. The experiment shows, moreover, 

 that the boundary of the vacuole is not a smooth surface, for a number 

 of crystals do not follow the shortest path, but glide down along the 



^ Owing to the plasmic sti-ucture, the vacuoles may at first sight appear to be rod-like 

 in shape. 



