Structural Dijferentiation of Cytoplasm 



105 



afford confirmatory evidence of their composition by staining with 

 lipoid soluble dyes such as chrysoidin and by blackening with osmic 

 acid. These phenomena indicate that the film-forming substance is 

 rich in lipoids as compared with the protoplasm as a whole. 



The lamellar type of structure, of which there is optical evi- 

 dence in myelin figures and in the dehydrated surface film of some 

 cells, is much more highly 

 developed in irreversibly 

 differentiated structures 

 such as chloroplasts and 

 the myelin sheath of nerve. 

 A nerve fiber may be re- 

 garded as a sort of cry- 

 stallized filamentous pseu- 

 dopodium in which the 

 axon or endoplasm is 

 largely a bundle of parallel 

 protein fibrils and the 

 sheath or ectoplasm con- 

 sists of concentric sheets 

 of orientated lipoid mole- 

 cules alternating with 

 sheets of protein. 



Although it requires ex- 

 ceptional conditions to 

 produce visible double re- 

 fraction of ordinary proto- 

 plasmic films, molecular layering may be present at all times toward 

 their surface where the conditions for orientation operate more 

 strongly. (Contrary to what we might except, however, the usual 

 lipophilic adhesion of the cell surface would seem to indicate that the 

 hydrocarbon ends of the lipoid molecules point outward.) A closely 

 packed, superficial lipoid film is, of course, in harmony with the gen- 

 eral conception of the protoplasmic surface derived from the study 

 of permeability. The properties of the hypothetical plasma mem- 

 brane probably belong to an organized film of molecular dimensions 

 (perhaps including proteins as well as lipoids) rather than to the 

 ectoplast as a whole. 



As to the colloidal state of the isotropic film substance, its appar- 

 ent immiscibility with water points to a coacervate rather than a 



Fig. 1. Diagrammatic cross section of the 

 cytoplasm of a plant cell showing differentia- 

 tions, w — cell wall, e — ectoplast or proto- 

 plasmic surface film, pg — plasmagel layer or 

 cortical endoplasm. ps — plasmasol or liquid 

 endoplasm. k — kinoplasm. t — tonoplast. v — 

 vacuole, s — transvacuolar strand, m — mye- 

 lin processes, p — plastid. 



