413 



fibrils can yet be observed. Sometimes the muscle fibrillae, 

 even before losing their intra-columnar grouping, may show 

 indications of striations. Sometimes muscle fibres may even 

 be observed, one end of which has undergone striation, while 

 at the other end striations have not yet developed (fig. 123). 

 The visible changes in the development of the contractile 

 part of these muscles during the rest of pupal life consists 

 in a greater strengthening of the striations and the develop- 

 ment of Krause's membrane. 



Meanwhile the nuclei have moved from within the muscle 

 to the surface, where they lie often in quite prominent masses 

 of uncontractile cytoplasm (fig. 123). The interstitial sub- 

 stance of the muscle fibres seems to be produced by the only 

 partial fibrillation of the syncytical columns. 



The outer walls of the fused mass of myoblasts remain 

 as the sarcolemma. 



The development of the muscle insertions is quite simple. 

 Each syncytial column, before fibrillating, fuses with a pro- 

 cess, several of which may be formed, from the adjacent 

 integument. In the late larva these processes are quite long, 

 but already in the fresh pupa they have begun to retract 

 (fig. 122), evidently exerting a pull on the muscles shortly 

 before these differentiate. They soon shorten to the thickness 

 of the other integumental cells, and during the third and 

 fourth day chitinise, giving the muscle insertions the appear- 

 ance of being inserted directly on the chitinous exoskeleton. 



By this process the eight pairs of pharyngeal dilators are 

 produced (fig. 124). In structure they are intermediate 

 between that of the abdominal muscles on the one hand, and 

 that of the muscles of the mouth appendages and of the leg 

 muscles on the other. It would seem, indeed, that these 

 muscles have been evolved from muscles which once resembled 

 the pharyngeal dilators. 



The development and structure of the muscles of the 

 mouth appendages and legs, and others similar to them, must 

 now be considered. 



(4) The Muscles of the Mouth Appendages. 



The development of these muscles illustrates a mode of 

 formation which differs somewhat from that observed in the 

 other muscles above described — a method of formation which 

 is to be observed also in the muscles of the legs and of the 

 ovipositor. 



Even in the earliest larvae scattered embryonic cells, 

 with clear cytoplasm and large "vesicular" nuclei, may be 

 observed in the ventral portion of the head, in the neigh- 

 bourhood of the mouth appendages or their imaginal discs. 



