BREED: METAMORPHOSIS OF THE MUSCLES OF A BEETLE. 373 
life are the fibrillae of the larval muscles entirely dissolved. There 
seems to be no increase in the number of muscle fibres by longitudinal 
division, and the nuclei were not observed to divide amitotically, as in 
the other metamorphosing muscles. The usual tracheal cells are found 
accompanying these muscles. 
5. The degeneration of the larval muscles is entirely chemical, there 
being no evidence of phagocytosis. In the early pupa, there com- 
mences a gradual atrophy of the muscle substance, during which the 
muscle is partially divided into longitudinal strands. The nuclei show 
no evidence of degeneration until practically all other parts of the 
muscle have disappeared. They then undergo a typical chromatolysis. 
This happens in the late pupa. Occasionally, tracheal cells are found in 
the fissures formed by the breaking up of these muscles, 
In those cases which presented transitional conditions between degen- 
eration. and metamorphosis, the muscles underwent changes exactly 
similar to those of the metamorphosing muscles, until the stage was 
reached where the reconstructive changes begin. Then the degenerating 
muscles seemed to lack the stimulus to start this reconstruction, and, 
therefore, continued to atrophy, and finally disappeared at the same time 
and in the same manner as the more typically degenerating muscles. 
6. The histological changes of the muscles of new formation in the 
pupa were observed principally in the leg muscles of Bruchus. These 
muscles are formed from spindle-shaped mesoderm cells found in the 
larva at the bases of imaginal folds which represent the legs. These 
cells probably are derived from the embryonic mesoderm. In the 
young pupa these mesoderm cells form the muscle fibres, each cell possibly 
giving rise to a single fibre. In the youngest stage in which the muscle 
fibres can be distinguished with certainty, it is evident that there are 
two kinds of cells in this mass: one, the mesoderm cells which form 
the muscle fibres ; the other, tracheal cells which form the tracheae of the 
leg. The latter are presumably derived from the same source as the 
tracheal cells of the rest of the body, that is, from the intracellular 
tracheoles of the resting larva. These cells may be distinguished as 
mesenchyme. 
III, AppITIONAL. 
1. Incidentally some other points have been noted. The musculus 
episternalis of the metathorax, whose function former authors had sug- 
gested to be that of an expiratory muscle, was discovered not to have 
this function. In the imaginal form of Thymalus, the pair of episternal 
