breed: metamorphosis of the muscles of a beetle. 355 



which {cl.tr.) do not show their tracheal nature in the least, these 

 forming a direct transition to the tracheal cells of the previous stages 

 {cl. tr.y Figures 14, 19, etc.). The processes of these cells are embedded 

 in the muscle substance, and even some of the cells {d. tr.^) may be 

 entirely embedded in the muscle. All through the substance of the 

 muscle are found the processes {pre.) of these cells detached from the 

 cell body by the plane of the section. Some of these processes are solid, 

 but most of them are already tubular tracheoles, which show prominently 

 in the sections because their walls stain deeply. They may be seen 

 better in the more enlarged representation (Figure 32, pre). This 

 penetration of the wing muscles by the tracheoles has long been known, 

 but their development has never before been described. A similar 

 development of the intracellular tracheoles in other parts of the body has 

 been noted in several cases. 



It is probable that some of these tracheal cells become leucocytes at 

 about this period. Certainly the large vacuolated leucocytes which have 

 persisted from the larva, such as are shown in Figure 51, leucyt. 

 (Plate 7), disappear in old pupae, and their places are taken by smaller, 

 less vacuolated leucocytes which resemble the tracheal cells. These 

 new leucocytes grow in size, and soon are characteristically vacuolated 

 (Figure 36, leu'cyt.). 



The finer structure of the muscle substance at a stage corresponding 

 to Figure 21 (Plate 6) is shown in Figure 32. The fibrillae are much 

 more numerous than before (Figure 29), and show more plainly in cross 

 section, while the amount of stainable sarcoplasm between them is 

 relatively less, so that the muscle as a whole stains fainter than before. 

 In longitudinal sections the fibrillation is plain, but no cross striation is 

 visible. In none of my sections of pupae does the cross striation show 

 in these muscles, but it appears in a series of sections of an imago a few 

 hours old (Figure 31), so that possibly this striation is formed during 

 the last stages of pupal life. 



In the stage shown in the longitudinal section the muscle nuclei 

 (Plate 7, Figure 35, wZ.^) are still dividing amitotically, but in the 

 somewhat older stage, shown in cross section only (Figure 21, Plate 6), 

 amitosis is rare. The nuclei in this older stage are numerous and are 

 scattered throughout the substance of the muscle. They are short oval 

 in form, the elongated nuclei of the preceding stages having disappeared 

 entirely. 



y. Imaginal Period. The structure of the wing muscles of insects has 

 been described so well by various authors that it need not be repeated 



