breed: metamokphosis 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 traclieae 

 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, i. 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, ia 

 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 ai'e 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. 



/?. Pupal or Reconstructive Period. The time of pupation agrees 

 closely with the change from destructive to reconstructive changes in 



