1 CYTOPLASM 183 



werk). To my mind this depends on whether we have to do with a 

 plasma gel or a plasma sol (p. 1 65). In the first case there must be some 

 interaction between the invisible fibrils, whereas in the second case 

 they may be independent of each other. 



The fibrillar theory has been developed partly on the basis of fixed 

 ■structures. This derivation is not as unreasonable as has often been 

 suggested, since on fixation the submicroscopic or amicroscopic 

 strands of the cytoplasm combine into coarser strings by directed 

 ^coagulation and can thus become microscopically visible. It is only 

 because the cytoplasm actually possesses a thread structure, that the 

 good fixations obtained by cytological micro-techniques are possible. 

 In this process the molecular framework may shrink, be coarsened, 

 <leformed and disturbed, but a clear-cut separation of coagulum and 

 serum as in the case oi protein solutions of like concentrations (milk, 

 fibrinogen) does not occur. 



The protoplasmic framework, which proves to be very stable with 

 respect to hydrolyzing substances, may be identical with Reinke's 

 plastin (1881). The latter represents the insoluble and not easily di- 

 gestible part of the cytoplasm; both these properties belong to the 

 cytoplasmic protein framework. On drying, it becomes still less di- 

 gestible, which may be connected with the fact that the strands of the 

 framework combine into coarser strings, as in fixation, and then are 

 less accessible to the destructive enzymes. 



The introduction oi plastin as a collective concept for the entire 

 protein frame is very convenient for describing these conditions. 

 Although Reinke did not think of a network, its properties tally well 

 with the characteristics given by him. The original concept "plastin" 

 has no chemic?l meaning, for it is characterized only in the negative: 

 insolubility, indigestibility, absence of phosphatides and lipids; in 

 short, what remains if everything sensitive to mild physico-chemical 

 intervention has been removed. Reinke's expression plastin is there- 

 fore a morphological concept Hke chromatin in the nucleus, and as such 

 is almost indispensable for purely descriptive purposes. For this reason 

 it is regrettable that Kiesel (i93°)' ^^^^"^ having isolated certain 

 protein-Hke skeleton substances from the plastin of slime moulds (in 

 Reinke's original sense), has applied the name "plastin" to a well- 

 defined protein compound. It is better to give a new name to these 

 chemically defined substances, and to maintain the plastin concept in 



