PHYSICS OF SEGMENTATION. 113 



water in the HC\-Asterias method, /. c., if another potential dif- 

 ference exists than in the cell, the latter must in turn do work ; 

 its two potentials accordingly decrease, the extensity factors in- 

 crease, water is absorbed, and as a result of new localized chemi- 

 cal and osmotic differences in the egg plus accompanying elec- 

 trical changes, the constricted form of cleavage appears. 



(Semipermeable Membranes.') 



In the above presentation the assumption is implicitly made 

 that the surface of the cell acts as a semipermeable membrane, 

 so that, first, only specific ions can enter, and second, others, 

 those within the cell cannot make their exit, while water can pass 

 either way. Only on these conditions can osmotic pressure be 

 truly exerted. 



The question accordingly arises, if this possibility is done away 

 with if the so-called membrane is due to, or identical with, only 

 the surface tension of a fluid in which colloidal particles are in 

 solution. The answer to this on the basis of the evidence at 

 hand and to accept the position generally taken is negative, but 

 again brings us to the matter of chemical organization. Rhum- 

 bler 1 has shown experimentally that particles in the surface are 

 not freely displaceable, nor do protoplasmic streamings to the 

 outside take place ; this can be accounted for only by assuming 

 a foam structure and a state of solution. This means that the 

 apparent membrane is a result of the forces of surface tension. 

 Furthermore, with very probable reasons for the surface being 

 chemically different to the interior as it is to that of the surround- 

 ing medium, and with the known fact of the difficult diffusibility 

 of colloidal particles, it is quite intelligible that at the same time 

 that a displacement to the interior is rendered difficult the specific 

 chemical nature of the surface film should prevent the passage 

 through it of some ions while allowing that of others, quite 

 analogous to the artificial semipermeable membranes of Pfeffer. 

 The same explanation would hold good for any other apparent 

 membranes, like that of the nucleus, within the cell whose mor- 

 phological differentiation can be established. 2 All these factors, 



1 Rhumbler, " Aggregatzustand u. Physikalische Besonderheiten des Zellinhaltes," 

 Zeitschrift fur allg. P/iysiologie, I., 3, '02. 



2 Cf. Hober, loc. fit., p. 47, for confirmatory evidence. 



