THE NERVOUS CYTOLOGY OF PEEIPLANETA ORIENTALIS. 353 



XVIII. A REVIEW OF THE TECHNIQUE. 



The most important result of this work, so far as methods are concerned, is, I 

 believe, the demonstration of the emphatic necessity of examining the cells, whenever 

 possible, in the fresh state. A control over all the rest of the technique, that can be 

 obtained in no other way, is thus secured. 



So far as fixing reagents are concerned, it seems to be sufficiently demonstrated 

 that the fluids in common use for this purpose may do grave damage to delicate 

 cellular structures. This damage, moreover, is, in great part, physical in nature; in 

 other words, it is largely due to osmosis and diffusion currents. If two aqueous solu- 

 tions of different osmotic tension are situated on opposite sides of a membrane, water 

 will tend to pass through that membrane from the solution of lower towards that 

 of higher osmotic influence, until enough has passed to render the osmotic values of 

 the two solutions approximately equal. A solution of a given osmotic power has 

 thus a dehydrating power over all solutions of a lower osmotic power than itself and 

 vice versa. The osmotic power of a solution depends, first, on the number of molecules 

 in a given volume, and, secondly, upon the dissociation of these molecules into their 

 ions. Proteids therefore in solution, since they have a very large and very heavy 

 molecule and never dissociate, have a much lower osmotic power than solutions of 

 equal percentage by weight of the simpler inorganic substances. Moreover, the pro- 

 teid substances in the cell-sap do not constitute a solution in the strict physical sense 

 of the term, but possibly occur in a semifluid condition, and under these circumstances 

 their osmotic influence is reduced to a minimum. 



Hence when a mass of protoplasm enclosed in a membrane is surrounded by a 

 fluid of greater osmotic power than that of protoplasm, the mass shrinks with a wrink- 

 ling of its membrane. If, on the other hand, the surrounding fluid is of lower osmotic 

 value than the protoplasm, the mass will swell. These facts are well illustrated by 

 the behavior of red-blood cells, which in a 2% solution of sodic chloride rapidly crenate, 

 while in distilled water they gradually swell and finally burst, probably chiefly on 

 account of the inorganic salts which they contain. The same phenomenon in 

 plant-cells is used to compare the osmotic values of different solutions. 



The problem in the nerve-cell of the roach is not quite as simple, for here 

 we have to deal with a mass of practically naked protoplasm, containing a definite 

 reticulum, and, embedded in it, a second mass surrounded by a membrane. It is 

 to be expected that the outer mass, being more exposed both by reason of its position 



