BREED: METAMORPHOSIS OF THE MUSCLES OF A BEETLE. 369 
imaginal muscles. At no stage do these metamorphosing muscles lose 
their identity, so that a dissolution of these muscles and a survival of 
their nuclei only, is impossible. 
Berlese’s mistake may be easily explained, however. He has neglected 
entirely the study of the anatomical changes; these would have immedi- 
ately revealed the falsity of his view. Moreover, he is unfortunate in 
his choice of the adductor of the mandible, as a muscle in which to study 
these changes. This muscle is composed of numerous fibres (50 in the 
larva, 250 in the imago of Thymalus), so that it is impossible to follow 
any particular one of them in its development. When the destructive 
changes in the metamorphosis of this muscle are completed, there re- 
mains simply a confused mass of these fibres still retaining their nuclei, 
with numerous spindle-shaped cells scattered between the fibres, pre- 
cisely as Berlese describes and figures (:02°, p. 65, Fig. 253). His 
mistake arises from his imagining that spindle cells are derived from the 
muscle nuclei, a mistake very easily made. In some of the beetles 
which I have examined, the difference between these cells and the 
muscle nuclei is not obvious at first sight. In Thymalus, however, there 
can be no doubt of a difference between them at all stages. As already 
shown, the spindle cells develop from tracheae and into tracheae, while 
the muscle nuclei persist as they are in the undifferentiated sarcoplasm 
and form the imaginal muscles. The conditions which Berlese shows in 
his second figure (Fig. 254) are different from anything observed in 
Thymalus. That all the cells pictured in this figure are of the same 
nature, is open to question. It has also been shown that there is no 
need of supposing a derivation of complete cells from nuclei alone, as 
Berlese has done. This assumption itself is enough to shake one’s 
confidence in his views. 
He also lays great stress on the simplicity of his idea, and the fact that 
he has been able to make it apply in every case which he has studied. 
But there may be a fault in too great simplicity, as well as in too great 
complexity. The reasonableness of the ideas of the present paper, as 
contrasted with those of Berlese, may best be shown by tracing what 
may have been the phylogenetic development of these muscular changes. 
It is fair to assume that in primitive insects the muscles ‘were the 
same in number, function, and position, when the larva escaped from the 
egg, as they were when the imaginal form was attained, since there 
doubtless was little difference between the two stages except in size. 
Now, in the development of such primitive insects into hemimetabolic 
forms, and the development of these into holometabolic forms, it has 
