BREED: METAMORPHOSIS OF THE MUSCLES OF A BEETLE. 353 
the walls of the tracheae, the tracheal cells, and in the cells of the hypo- 
dermis, the latter being, of course, the tissue from which the tracheae 
were derived. Few of the other tissues show mitosis, amitotis being the 
method of division in both leucocytes and muscle nuclei. Moreover, 
there is little chance of confusing the tracheal cells with leucocytes, as 
the latter are readily distinguishable by their more rounded form and 
finely vacuolated cytoplasm, which does not stain as deeply as the cyto- 
plasm of the tracheal cells. The sudden appearance of the tracheal cells 
in all parts of the body at once, precludes any possibility of a local place 
of origin, such as the base of the wing, etc. Finally their fate, 1. e., 
development into tracheae, indicates their origin from tracheae. 
The question might be raised, whether or not these cells are the active 
agents in the splitting of the muscle into strands, This can scarcely be 
so, because the earlier the stages in the changes of these muscles, the 
fewer are these cells in the spaces between the strands. Moreover, in 
the earliest stages there are numerous fissures in which there are no 
tracheal cells. 
The relationships of these tracheal cells to the mesenchyme, mesoderm, 
embryonic cells, myocytes, etc., which other investigators have found in 
connection with the postembryonic development of insects, cannot be 
entirely settled. The tracheal cells are doubtless the same as the 
spindle cells of Deegener. It is also probable that they are the same 
as the so-called myocytes of Berlese ; at least, the same as those that he 
has described for Coleoptera. That entirely different kinds of cells have 
been described under these various terms, is almost certain. For my- 
self, I am disposed to think that there are present during the metamor- 
phoses of holometabolic insects, two distinct kinds of embryonic cells, 
which resemble each other in form and structure, but which have differ- 
ent origins and fates. One kind might properly be called mesenchymal ; 
these are cells which arise singly from the tracheae or hypodermis and 
rise to tracheae, leucocytes, and other related tissues. Such cells are 
to be expected in most cases. The other kind may be called mesodermal. 
Their origin is not established as yet, but probably they are derived 
from cells of the embryonic mesoderm which persist until pupal life. 
They give rise to muscles and possibly other tissues in the pupa and are 
found principally in those insects in which muscles are newly formed 
during pupal life. There are many facts to support such a view, but it 
cannot be definitely proved with the material at hand. 
B. Pupal or Reconstructive Period. The time of pupation agrees 
closely with the change from destructive to reconstructive changes in 
