401 



a memhrane. At any rate, Krause's "membrane" is devel- 

 oped by the individual fibrils, and, if adjacent "membranes'' 

 do unite, then the structure is secondarily, not primarily, 

 a membrane. I shall refer to this again in connection with 

 the structure of the adult muscles. 



In connection with the ''transverse" striations, a curious 

 fact was noticed which has not, apparently, been hitherto 

 recorded. The striations do^ not run transversely across the 

 muscle fibre; on the other hand, the fibrillae are so disposed 

 that their thickenings in the fibre as a whole are disposed 

 in the form of a double spiral (fig. 101). This double spiral 

 is not always visible in longitudinal sections, as the muscle 

 fibre may have been so cut as to show only a portion of it; 

 under these circumstances it will appear either as true trans- 

 verse striations or as a single spiral. However, in moderately 

 thickly-cut sections the double spiral is almost always clearly 

 visible. Moreover, it is possible to focus on top of a ''trans- 

 verse" striation, and beginning at one end and focussing 

 alternately downwards and upwards, to travel right along the 

 spiral, and finally arrive at the other end of the fibre. Also, 

 after following a spiral striation through a, single turn one 

 arrives, not at the succeeding striation, but at the second in 

 advance, showing the double nature of the spiral. By no 

 conceivable bending or twisting of the muscle fibres could 

 true transverse striations be thrown into this form, and the 

 question of artefacts can be discarded; moreover, the double 

 spiral may be detected in entire muscle fibres if these have 

 been sufficiently stretched to allow the spiral on the distal 

 side of the muscle to show through the thickness of the fibre. 



Krause's "membrane," of course, is likewise disposed in 

 a double spiral. 



The sarcomeres of either end of the muscle frequently 

 have only one "Krause's membrane"; the outer end of the 

 sarcomere being in this case inserted into the cuticle of the 

 larva. Sometimes the fibrils can actually be traced into the 

 cuticle, where tliey spread out a little to procure an extra 

 hold (fig. 127). At other times they are inserted on to the 

 terminations of integumental cells (fig. 100). The essentially 

 integumental origin of the muscle insertions will be referred to 

 later, in connection with their development. 



Others of the fibrils, however, do not become inserted into 

 the cuticle, but travelling across the border of the segment 

 join fibrils from the next muscle of the same longitudinal 

 band, forming a very powerful "Krause's membrane" at the 

 junction (fig. 126). Wlien a muscle is examined in surface 

 view these crossing fibrils are clearly seen, giving the muscles 

 a particularly frayed appearance at their extremities. 



