24 



FUNDAMENTALS OF CYTOLOGY 



enter or leave the protoplast and the rate of their movement are deter- 

 mined not only by the nature of the substances, but also by the character 

 of the membrane. The membrane is semipermeable, i.e., it may allow 

 the solvent but not the solute to pass, and physiologists are attempting 

 to account for this significant property and its fluctuations in terms of 

 physic ochemical constitution. 



At the boundary between the cytoplasm and each of the other main 

 parts of the protoplast to be described below there is a special membrane 

 of some kind, so that interchanges there, too, are determined b}^ regional 



.*. 





The cell has numerous prolongations, 



^,/_:?.#:^ / #■ 



Fig. 12. — Motor nerve cells in spinal cord of ox. 

 the nerve fibers, which are embedded in a tissue known as neuroglia. Fine striations in 

 the cytoplasm are neurofibrils (compare Fig. 13). {Courtesy of General Biological Supply 

 House, Inc., Chicago.) 



differentiations involving the cytoplasm. To what extent the special 

 alterations at such an interface involve changes in the materials on either 

 side of it is not W'cll known. There are reasons, both experimental and 

 theoretical, for the view that a membrane formed where two different 

 fluids meet arises b}^ a local modification of both fluids. The membranes 

 within the protoplast, notably that around the nucleus, would in this 

 sense be double structures. Precipitation membranes composed of new 

 components formed by chemical interaction of two fluids probably 

 play a minor role. It should always be borne in mind that the behavior 

 of the protoplast depends not onlj^ upon the general composition of its 

 principal parts, but also upon further special modifications in areas where 

 two unlike components meet. 



