August 23, 1912] 



SCIENCE 



249 



condition from segment to segment can vary- 

 only in the concentration and state of aggre- 

 gation of its colloid material. There are indi- 

 cations that the colloidal particles (probably 

 submicrons) are more densely aggregated in 

 the dim bands and less densely in the light; 

 in this case the interstitial spaces must be 

 smaller in the dim and larger in the light 

 bands; i. e., the proportion of displaceable 

 fluid material is greater in the light bands, 

 the latter, so to speak, having the looser tex- 

 ture of the two. Such a conception explains 

 many characteristic observations, such as the 

 beaded form shown by fibrils under certain 

 conditions; the greater compressibility of the 

 isotropic segments (indicating a greater 

 fluidity), as shown by the experiments of 

 Haycraft, who obtained impressions of the 

 segmented structure by pressing living fibers 

 against collodion; the facts that the double 

 refraction appears rather to diminish than en- 

 tirely to disappear in the light bands, and 

 that adsorbed coloring materials, such as 

 hematoxylin, though taken up by the whole 

 fibril, are extracted most readily from tbe 

 isotropic segments, just as though the 

 adsorbing surface were there relatively less. 

 Hiirthle also compares the material com- 

 posing the fibril to a gel; he regards this 

 as chemically homogeneous throughout, but 

 as differing in its physical condition in alter- 

 nate segments. He adduces the analogy of 

 the nodes in a stretched vibrating string, im- 

 plying that the differences between the alter- 

 nating segments are purely physical and in- 

 dependent of differences in chemical compo- 

 sition. Although it is doubtful that marked 

 differences in the colloidal aggregation-state 

 could exist between adjacent segments with- 

 out at least some quantitative differences of 

 chemical composition due to differences of 

 adsorption, etc., yet the essential conception 

 of the fibril as essentially homogeneous in its 

 chemical composition is not altered by recog- 

 nizing the existence of a segmented structure 

 of the above conceived kind. 



On this view the fibril is to be regarded as 

 a column of colloidal material having the es- 

 sential properties of a gel — i. e., a system in 



which the colloidal particles cohere; the par- 

 ticles, however, are not homogeneously distrib- 

 uted in this gel, but are relatively densely 

 aggregated in the anisotropic segments and 

 loosely in the isotropic ; in the latter the inter- 

 stitial fluid-containing spaces are therefore 

 relatively larger. It should be noted that in 

 all muscles the isotropic segments are shorter 

 than the anisotropic ; also that the best photo- 

 graphic evidence indicates that the actual 

 shortening is confined to the anisotropic re- 

 gions of the fibril, i. e., to the regions where 

 the colloidal particles are the most closely 

 aggregated. The significance of this will be 

 apparent later. 



The considerations urged in the preceding 

 paragraph relate to the individual fibril and 

 do not explain the lateral coherence of the 

 fibrils. It seems clear, from the fact that the 

 striation is preserved during contraction, that 

 transverse connections must exist between ad- 

 jacent fibrils, otherwise the latter would cer- 

 tainly undergo relative displacement more 

 frequently than observation shows to be the 

 case. But the histological evidence of such 

 cross connections is imperfect, although the 

 view seems well founded that the appearances 

 known as Hensen's and Krause's lines (M 

 and Z lines) correspond to regions where ad- 

 jacent fibrils are laterally conjoined. This 

 view is supported by Heidenhain on the basis 

 of numerous observations on fixed muscle of 

 various kinds; and a continuity of the Z-lines 

 from fibril to fibril, even when the latter are 

 separated by some distance, is clearly ap- 

 parent in some of Meigs's photographs. Both 

 of these appearances, especially the Z or inter- 

 mediate line, have been interpreted as mem- 

 branes subdividing the fibrils at segmental 

 intervals; but they seem to lack the continu- 

 ity which such a conception demands. A 

 continuous membrane would show definite 

 optical appearances due to reflection of light 

 at its surface. The so-called /-lines, which 

 suggest such reflection and hence support this 

 conception, are variable and often unapparent. 

 On the other hand, the existence of narrow 

 cross-connections between adjacent fibrils 

 would undoubtedly account for the observed 



