534 HENRY M. BERNARD, 



pous layer alone contracts, concludes that in the muscle fibril, the 

 isotropous layer represents the attracting substance towards which 

 the anisotropous substances retreat, "as a pseudopodium is drawn in 

 towards the nucleus". Schäfer, on the other hand, explains the con- 

 traction of the fibril as the retreat of the isotropous substance into the 

 anisotropous substance as the "hyaloplasm retreats into the spongio- 

 plasm". On this point of difference, as to which layer attracts the other, 

 I have no doubt, as will be seen from what follows, that Schäfer's 

 view is correct. 



I had, then, the following questions to try and answer: 

 1) Can the phenomena presented by the muscle fibrils in the 

 different phases of contraction be explained as the attraction of the 

 isotropous layer towards, and its absorption by, the anisotropous 

 layer? 2) Is this the only factor in the movement of contraction? 

 3) Can the anisotropous layers be considered as nuclear, attracting as 

 such the clear protoplasmic substance, Verworn having shown nuclear 

 attraction to be a probable explanation of the movement of contraction 

 in living Rhizopods? 



I. 



Selecting muscles (Fig. 1, 2) which showed different states of ex- 

 pansion and contraction along their lengths, so as to be able to make 

 trustworthy comparisons, when possible, between the "elements" of the 

 same fibril, or at least between the "elements" of fibrils of the same 

 muscle, I found: 



1) In the state of the greatest expansion the "elements" were often 

 thrown into a zig-zag (Fig. 5) ; this arrangement may be due to the hand- 

 ling of the animal after death, or it may indicate an abnormal degree of 

 active expansion in the death agony. Each "element" had the appear- 

 ance of being a spindle-shaped body with the staining matter con- 

 tracted into a spindle-shaped mass in the centre (Figs. 4, 5, 6, 7, and 

 part of 9). 



2) In the normal state, the fibril is to all appearance absolutely 

 cylindrical (Figs. 10, 11, 12, 13, and parts of 9 and 14). This appearance 

 is very difficult to reconcile with Haycraft's remarkable collodium 

 impressions ^). I cannot myself help thinking that these impressions are 

 due to differences of hardness along the fibril. But even if the striated 

 appearance be due to the form of the fibril, not only have these remark- 



1) On the minute structure of striped muscle &c., in: Proc. Roy. 

 Soc. London, 1891, vol. 49, p. 287. 



