THE STRUCTURE OF PROTOPLASM 



261 



of fluids (streaming, rounding up, etc.) and yet also possess 

 properties so essentially characteristic of solids (elasticity, etc.). 



The hypothesis of gel structure so far presented is now gener- 

 ally held by the chemists and rapidly winning favor with the 

 biologists as an interpretation of protoplasmic structure. H. R. 

 Procter regards gelatin jellies as built of a network of molecules 

 or of hnear aggregates (threads) of molecules. J. A. Wilson is of 

 a similar opinion, though limiting 

 the structural fibers to units of 

 atomic diameter {i.e., a single 

 molecule) or at least comparatively 

 small polymerized groups. The 

 long chains of amino acids satisfy 

 all necessary conditions for such 

 fibers. As already stated, the 

 terminal acid groups may unite 

 with the basic ones until a con- 

 tinuous network is formed. A 

 block of jelly would then become 

 a single molecule, after the manner 

 of crystals. R. H. Bogue pic- 

 tures the process of gelation from 

 a hot solution (of gelatin) to a 

 solid jelly as involving the for- 

 mation of catenary threads by the 

 union of individual molecules end 

 to end. The production of soap curds is attributed by McBain 

 to similar units and forces. He ascribes the elasticity of curds 

 to a fine, filamentous structure. 



Structural Continuity in Protoplasm. — Having accepted the 

 work of chemists in regard to the structure of gels, and having 

 found this structure to be one of linear crystalline units forming 

 either a brush heap of fibers or a symmetrical arrangement of 

 fibers oriented end to end in parallel rows, our next task is to 

 see how far this hypothesis of structure will hold for protoplasm. 

 That it meets certain qualifications has already been indicated, 

 but we can now give more substantial evidence. 



Fibrous structures often appear in cells when these are fixed 

 (killed) and stained (Fig. 123); they must have arisen through 

 aggregation of smaller fibrous units. But we need not go to 



Fig. 139. — Protoplasm being 

 torn by a microneedle (the 

 large black spot) and thereby 

 showing its fibrous structure and 

 the formation of highly elastic 

 strands. 



