I CYTOPLASM l6l 



secondary valency bonds; p^ influences the heteropolar or salt bonds, 

 and the redox potential is capable of intervening, either as a con- 

 structive or as a destructive factor, in the homopolar valency bonds or 

 bridgelike bonds. It is therefore very difficult to explore the structure 

 of the cytoplasm experimentally, for it is scarcely possible to vary only 

 a single one amongst these four factors, keeping the three others 

 rigorously constant. A change in the temperature or the salt concen- 

 tration will often cause changes in pn and tn, which in their turn are 

 interdependent. For this reason one can never be sure in an experiment 

 whether some measure has not affected other tj^pes of bonds besides 

 the group of points of attachment which one wished to investigate. 

 In spite of the fact that it is practically impossible to keep the four 

 types of junctions as neatly apart as in theory, this scheme gives an 

 idea of the various kinds of bonds which by their harmonious col- 

 laboration are responsible for the remarkable structure of protoplasm. 

 In the case oi fixation, the aim is to preserve the molecular arrange- 

 ment as true to life as possible. This can never be done perfectly, since 

 the usual means of fixation affect quite different categories of junctions. 

 Alcohol has a dehydrating and hardening effect on the heteropolar 

 cohesive bonds. In order to counteract the accompanying shrinkage, 

 a swelline medium such as acetic acid has to be added. Its H-ions 

 lessen the contracting action of the alcohol by hydrolysis of hetero- 

 polar valency bonds and by maintaining a certain state of hydration of 

 the heteropolar cohesive bonds. Oxidizing fixatives like chromic acid 

 and osmic acid affect bridges which are sensitive to r^^ and thus solidify 

 the labile hompolar main valency bonds. The tanning action of formal- 

 dehyde may be due to its capacity to form bridges between neighbour- 

 ing polypeptide chains according to the same scheme as that which 

 governs the polymerization of oxymethylene. It is impossible to find 

 a fixation mixture which in no way affects the structure of the 

 labile cytoplasm. In spite of this, fixations which have been carried 

 out correctly cannot be compared with precipitations, since there is 

 no separation of phases, but only a coarsening of an existing structure. 

 It is shown by the dyeing experiments carried out by Drawert (1957) 

 with varying p^ of the imbibing liquid, that the molecular framework 

 after fixation still contains acidic and basic groups capable of dis- 

 sociation, although these groups are no longer screened off but are freely 

 accessible to dyes. This is why fixed cells can be stained easily, whereas 



