BREED: METAMORPHOSIS OF THE MUSCLES OF A BEETLE. 355 
which (cl.¢r.) do not show their tracheal nature in the least, these 
forming a direct transition to the tracheal cells of the previous stages 
(cl. tr., Figures 14, 19, etc.). The processes of these cells are embedded 
in the muscle substance, and even some of the cells (ci. tr.*) may be 
entirely embedded in the muscle. All through the substance of the 
muscle are found the processes (prc.) 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, prc.). 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, lew’cyft. 
(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, lew’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, nl.") 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 
