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. Additional. 



1. Incidentally some other points have been noted. The musculns 

 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 



