536 HENRY M, BERNARD, 



a) as a general mixture, which I think occurs in Crustacean 

 muscles, accounting thereby for the slight lengthening of the aniso- 

 tropous layer, or, 



b) where more rapid contractions or expansions are required e. g. 

 in the wing muscles of Flies, by the rapid flowing in and out of the 

 isotropous layers into regular pits in the anisotropous substance, such 

 as are clearly revealed in Schäfer's photographs. From this point 

 of view, Schäfer's pits would be a natural and necessary specialisation 

 of the simpler conditions in which the substances freely mix. 



4) In the final stage of contraction, the whole compartment has 

 shortened to the utmost. The stained substance is found massed at 

 each end of the "element", which ends are somewhat swollen, Hensen's 

 disc is become so pronounced that the isotropous and anisotropous 

 layers appear to have changed places ("Umkehrungsstadium" ; Fig. 18, 

 19, 20, 21, 22). This swollen condensed appearance at the ends of 

 the "elements" might be explained as due to the close union of the 

 isotropous and anisotropous substances, which in the transition stage 

 had been but loosely mixed. That the isotropous substance has been 

 completely absorbed by the anisotropous follows from the fact that 

 whereas the former has completely disappeared, the latter has not 

 materially changed its length (cf. diagram Fig. 24). The swelling and 

 condensed staining at the ends of the "element" appear to me quite 

 inconsistent with the persistence of the isotropous layer per se 

 (Engelmann). If this explanation of the phenomena is correct, the 

 inverse stage should be most pronounced in those less dififerentiated 

 fibrils in which the isotropous and the anisotropous substances freely 

 mix, i. e. in which the staining element in the anisotropous substance 

 has no definite structure, but is free to move and mix with the iso- 

 tropous substance. On the other hand, in the highly specialised wing 

 muscles of Flies, Schäfer's photographs show that the anisotropous 

 layer has a definite structure; hence it simply draws in the isotrop 

 ous substance without itself becoming disorganized and massed at the 

 end of the element. There would therefore in this case be no swelling 

 round Krause's discs, but rather a sinking in. Schäfer, further 

 speaks of a slight deepening of Hensen's disc. But there is certainly 

 nothing so pronounced as that observable at this stage in Crustacean 

 muscles (Fig. 21 and 22). This slight deepening of Hensen's disc in 

 these highly specialised wing muscles, however, is very interesting, as 

 it serves to link the phenomena together. 



