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from fibril to fibril across the interfibrillar space and not confined to the fibrils, 

 as Rutherford supposed. Its constant position between successive sarcous 

 elements shows that it, also, must be arranged not as a series of successive discs, 

 but as a double helicoid — complete, however, and not composed of minute 

 elements (sarcous elements), as in the case of the "striations." The membranes 

 of Krause, as Krause himself observed, are always inserted upon the sarcolemma 

 (see figs. 1 and 2, pi. xi.). 



The quantity of sarcoplasm between the fibrils varies considerably ; in the 

 muscles of very sluggish insect larvae it is present in only quite small quantities 

 (fig. 1, pi. xi.j. In the more active vertebrate muscles it is present in larger 

 quantity, especially just beneath the sarcolemma. In the powerful wing-moving 

 muscles of insects it is present sometimes in very large quantities ; and in the 

 most active of all these muscles, viz., the wing-moving muscles of large wood 

 moths or heavy beetles, it may be much more prominent than the fibrils them- 

 selves (see fig. 9, pi. xii.). In some insect wing muscles the sarcoplasm may, 

 in fixed preparations, at any rate, show a curious architecture, strongly resembling 

 the striations of other muscles ; so prominent is this, that the true striations 

 are not seen at first glance, and can only be observed by carefully examining 

 the individual fibrils (fig. 9, pi. xii.). 



When the individual sarcomeres are examined in the uncontracted condition 

 each is seen to be a cylindrical structure (figs. 1 and 3, pi. xi.), often bulged 

 in the middle (fig. 6, pi. xii.). At either end of this cylinder is the delicate 

 Krause's membrane, and usually in close contact with this is to be found, even 

 in insect wing muscle, a very minute amount of dark-staining material similar 

 in appearance to the material constituting the striation, and corresponding to 

 the dots figured by several authors in close connection with Krause's membrane. 

 I shall speak of it as the residual hyaloplasm. In the greatly hypertrophied 

 larval insect muscles a number of extremely delicate streaks of dark-staining 

 material are just visible under the highest magnifications (fig. 3, pi. xi.), con- 

 necting this residual hyaloplasm to the striation, and staining similar to it. 

 The interpretation of this is that there are in the clear part of the sarcomere 

 minute channels (similar in appearance to those observed by Schafer within the 

 dark-staining sarcous elements of wing muscle) which connect the residual 

 hyaloplasm, with the striation, or, as I shall call it, movable hyaloplasm. 



Now if the individual sarcomeres are examined successively along a con- 

 traction wave, the actual contractile process within the sarcomere can be 

 observed, and it can be most clearly seen that a process the very opposite to 

 that maintained by modern histologists, actually takes place. As we pass down 

 the contraction wave the striations become fainter and fainter and the dark 

 material accumulates more and more around Krause's membrane (see fig. 15, 

 pi. xiii.), and the reversal of striations is actually seen to he produced zvithin 

 the sarcomeres, and not by the interfibrillar sarcoplasm, as Schafer maintains. 

 In the greatly hypertrophied larval muscles the contracted state of the fibrils 

 can be most clearly observed (fig. 2, pi. xi.), and the sarcoplasm is seen to have 

 no relation whatever to the reversal that occurs ; it is a phenomenon confined 

 entirely to the fibrillae. 



How may this process be reconciled with the observation of Engelmann 

 that the relative positions of the singly and doubly refracting materials are 

 unchanged? In the first place, the opinion has been advanced by Imbert (1897) 

 that surface tension phenomena underlie muscular contraction; Bernstein (1901) 

 showed, however, that the decrease of area of the movable fluid, as required 

 on Imbert's view, is insufficient to account for the work performed during con- 

 traction, unless a further difirerentiation, which might be in the form of a 

 spongy network, occurs within the sarcous elements. Schafer has described 



