410 



muscle cell contents phagocytically, or whether they merely 

 absorb it through their permeable cell wall, I am unable to 

 say. Perhaps, after all, the only function of the pseudopodia 

 is to enable the myoblasts to crawl into the position they are 

 to assume in the mature muscle, just as the cells of the 

 imaginal integumental areas do (see above). 



It is scarcely necessary to remark that the myoblasts 

 in the above account have not been confused with leucocytes, 

 the activities of which are not altogether unlike those of the 

 myoblasts; leucocytes are considerably larger than myoblasts 

 and always have a characteristic nucleus. ~ 



After twelve to fifteen hours the myoblasts of each 

 muscle fibre have arranged themselves, one after the other, 

 in a row; the pseudopodia have entirely disappeared (fig. Ill) 

 and the cells are almost cubical in shape. In this condition 

 they remain for a long time, the only visible change being 

 that they first adopt a very regular arrangement, and in the 

 thirty-six hour pupa fuse to form a long columnar syncytium, 

 with the nuclei regularly arranged along it from one end to 

 the other (cf. fig. 115). But in the middle of the third day 

 of pupal life the developing muscle fibre begins to differentiate, 

 and first undergoing fibrillation, then striation, develops 

 eventually into the muscle as we see it in the adult. The 

 striations, as usual, are spirally arranged. Tlie nuclei occur 

 right in the middle of the fibre (fig. 116). It follows, of 

 course, that a single longitudinal abdominal muscle consists 

 of only one fibre. 



The nature of the muscle insertions will be referred to 

 later. 



Perez (1910) has observed the metamorphosis of the 

 abdominal muscles in CalUphora. He finds that the muscle 

 fibres lose their striations and fibrillations, and that even the 

 sarcolemma is added to the degenerate mass. He described 

 the myoblasts as entering the dead larval muscle fibre 

 apparently by amoeboid action. Here they lose their cyto- 

 plasm and increase by ,di^^ct_ division to form the syncitial 

 mass, which, on differentiating, produces a m*ature muscle 

 fibre. It should be remarked that the myoblasts have only 

 an extremely fine pellicle in Nasonia, and that if the degen- 

 erate larval cytoplasm is at all compact in consistency, as it 

 is in the pharyngeal and thoracic muscles, the myoblast 

 cytoplasm is hardly, or not at all, distinguishable, unless, as 

 is very often the case, the myoblast lies in a distinct vacuole 

 within the mass, part of which it has, apparently, been 

 absorbing. The degenerate cytoplasm of the abdominal muscles 

 is, however, so loose in texture that the cytoplasm of the myo- 

 blast is easily recognizable. The apparent loss of cytoplasm as 



