336 J. DAVIDSON 



from entering the vacuole. The behavior of the Spirogyra 

 cells, reported by Lepeshkin, can be explained by mechanical 

 disintegration, similar to the falling apart of a large drop of 

 fluid into a number of smaller droplets. 



A special feature of the evidence in favor of the existence of 

 special plasma-membranes is that it all deals with dead or injured 

 cells. 



Pfefferi*^ ^j^q jg firmly convinced of the existence of plasma- 

 membranes admits that the evidence in favor of this theory is 

 as yet insufficient. The question, according to him, would be 

 considered definitely settled, if it were possible to prove that 

 substances distributed in the internal mass of the protoplasm 

 appear neither in the vacuoles nor in the outside medium. 

 Evidence of this kind, however, is wanting. Hober^^ determined 

 the electric conductivity in the interior or erythrocites and 

 having found it to be many times greater than in the erythrocites 

 as a whole concluded that the plasma-membrane was imper- 

 meable to ions present in the protoplasm. Kite^^ on the other 

 hand came to the conclusion (on the basis of unpublished data) 

 that the property of selective permeability is possessed by the 

 entire cytoplasm. 



Nevertheless the existence of special plasma-membranes is 

 tacitly acknowledged by most physiologists, and many of the 

 hypotheses offered to explain the osmotic and diosmotic prop- 

 erties of the cell are based on this assumption. Pfeffer'^ how- 

 ever and, more recently, Lepeshkin^* do not necessarily associate 

 the properties of the cell with the existence of plasma-membranes 

 on the inner and outer boundaries of the cytoplasm and admit the 

 possibility that the entire cytoplasm may serve as a thick plasma- 

 membrane. It may be added, that, taking into consideration 

 Pfeffer's conception of the formation of the plasma-membrane, 

 it is hard to conceive how its existence could in any way influence 



1° L. c, p. 91. 



" Hober, Arch. f. d. Ges. Physiol. 133: 237. 19;0. McClendon 1. c, p. 117. 



12 Kite, G. L., The relative permeability, etc. Biol. Bui. 25: 1-7. 



" L. c. 



" L. c. 



