STRUCTURE OF CELLS. 29 



sent knowledge has already been given in the introductory 

 portion of this section. We hold that the external limiting 

 layers of protoplasm may undergo both chemical and physical 

 changes, and there will then be produced a membrane that, 

 compared with protoplasm, is of firm consistence. The recogni- 

 tion of a double contour line in the uninjured cell is indispen- 

 sably necessary to prove the existence of an investing membrane. 

 Briicke remarks in reference to this point, " The difference be- 

 tween the density of the surrounding medium and the cell, even 

 when no investing membrane is present, will cause the appearance 

 of a boundary line. But it is only from the presence of a second 

 contour that we are able to recognize a difference in density 

 between the investing substance and the contained material. 

 It is self-evident that the power of the instrument must not be 

 pushed beyond its natural limits by the employment of strong 

 oculars, as in that case a second contour line makes its appear- 

 ance, not due to the structure of the cell, but consequent on 

 defects in the optical apparatus we are using." If, moreover, 

 a double contour line is observed after the action of reagents, 

 no proof is afforded that a membrane existed during life. 

 Kiihne argued respecting the value of the double contour line 

 as an evidence of structure, in the following terms : " If an 

 amosba were to surround itself by a broad hyaline border, not 

 regularly defined internally, I should not be surprised if a 

 reagent like acetic acid, which made this border suddenly 

 shrink whilst under observation, were to cause the appearance 

 of two wrinkled, closely approximated contour lines ; and if I 

 knew that this border was previously mutable, I should not 

 believe the solid membrane, originating in the action of acetic 

 acid, was previously present as an investment of the outer 

 hyaline layer." 



Many cells of the integuments possess distinctly perceptible 

 membranes, as in the case of the mucous cells on the surface of 

 fresh-water fish, first described by Leydig, and in a series of 

 other analogous structures which F. E. Schulze* has collectively 

 designated cup or chalice cells. F. E. Schulze distinguishes 

 two kinds of cells, in one of which the membrane (theca) is 



* Max Schultze's Archiv, Band iii. 



F 2 



