170 



THE CELL AND PROTOPLASM 



ciated with this method which was used to 

 study many organisms. In all eases the 

 microscopically optically empty, hyaline 

 cytoplasm was seen to contain, according 

 to Taylor (1925), "hosts of ultra-micro- 

 scopic particles displaying exceedingly ac- 

 tive Brownian movement"; while Bayliss 

 (1920) speaks of "an immense number of 

 very minute particles shown by their bright 

 diffraction images ... in vigorous Brown- 

 ian movement, . . . scarcely possible to 

 distinguish separate particles on account of 

 their number. ' ' I wish to call attention, in 

 both cases, to the emphasis on "hosts of 

 particles ' ' and ' ' immense number. ' ' These 

 are the only data of even a semi-quantita- 

 tive nature we were able to find. 



These small points of light are diffraction 

 discs produced by minute particles which 

 have been estimated, for materials of high 

 density (Gaidukov 1906; Zsigmondy 1914), 

 to be as smaU as 50 A to 100 A in diameter. 

 They are small in comparison to mitochon- 

 dria or other particles just within the vis- 

 ible range. In fact, one mitochondrium is 

 large enough to contain about a million 

 particles of 100 A size, and obviously if 

 that were the case, a mitochondrium would 

 be rather extensive as a structure, built 

 from a million such units; but concerning 

 its structure we have no direct knowledge. 

 Particles of 100 A in size are down in a 

 range where dimensions have been investi- 

 gated by several methods, and a consider- 

 able amount is known about them. Unfor- 

 tunately, however, there has been relatively 

 little work done from which one feels safe 

 in saying that the particles investigated are 

 truly those which were actually known to 

 exist in the cell. 



Existence of these minute particles in the 

 living material is perhaps open to ques- 

 tion; the objection being that the disturb- 

 ance of the protoplasm by excessive heat 

 when the ultramicroscope is employed pro- 

 duces the particles as artifacts. Further 

 investigation is clearly desirable, but in the 

 meantime one is led to wonder at the amaz- 

 ing coincidence of particle size with the 

 range of microscopic visibility. It seems 

 incredible that the region below the visible 



and above the molecular should be free of 

 particles; in other words, that particles 

 built from molecules or otherwise of atomic 

 groups should have a lower limit in size 

 which happens to correspond to the visible 

 limit of the microscope. This opens many 

 questions of experimental interest. That 

 particles exist in the hyaline material seems 

 to be the only conceivable condition in the 

 light of ordinary molecular aggregation. 

 Analogy and reasonable inference will have 

 to be accepted until other methods are 

 devised. 



Chemical analyses have shown quite con- 

 clusively that a large proportion of these 

 particles have the characteristics of pro- 

 teins (Chibnall 1926; Pearsall and Ewing 

 1924-5; Shinke and Shigenaga 1933; Gi- 

 roud 1929 ; Milovidov 1928) . The particles 

 probably vary from visibility, or about 

 5000 A, down to 100 A in size. Svedberg 

 (1937; 1939) and others have shown with 

 the ultracentrifuge that proteins in dilute 

 solution may occur as particles of these 

 smaller sizes. They are spoken of as mole- 

 cules and have had assigned to them molec- 

 ular weights. Thus, a particle of about 50 

 A corresponds to one having a molecular 

 weight of about 36,000; while one of 80 A, 

 corresponds to one with a molecular weight 

 of 300,000 ; and one of 250 A, to a particle 

 with a molecular w^eight of about 6,000,000. 

 Those in the larger categories may in many 

 cases be broken down rather easily into the 

 smaller 36,000-size or even to sizes still 

 smaller (Svedberg 1937; 1939). 



Analyses from various sources show that 

 in its active state protoplasm contains 

 about 85 per cent water, and of the remain- 

 ing materials about two-thirds protein. 

 The proteins and water together make up 

 approximately 95 per cent of the living 

 material, and it seems highly probable that 

 the particles we have been discussing are 

 composed principally of proteins. This 

 gives to the molecules of these two sub- 

 stances considerable importance in any 

 attempt to comprehend the molecular 

 structure. During the past decade investi- 

 gations of various sorts have provided con- 

 siderable information concerning both of 



