TR AC HE AT A. 351 



tions and new formations arise in connection with the alimentary 

 tract, the nervous and muscular systems. 



The changes which take place in the true flies (Muscidae) are far 

 more complicated than either those in Corethra or in the Lepidoptera. 

 The abdomen of the larva of Musca becomes bodily converted into 

 the abdomen of the imago as in the above types, but the whole 

 epidermis and appendages of the head and thorax are derived from 

 imaginal discs which are formed within and (so far as is known) inde- 

 pendently of the epidermis of the larva or embryo. These imagiual 

 discs are simple masses of apparently indifferent cells, which for the 

 most part appear at the close of embryonic life, and are attached to 

 nerves or trachea;. They grow in size during larval life, but during 

 the relatively long papa stage they unite together to give rise to a 

 continuous epidermis, from which the appendages grow out as pro- 

 cesses. The epidermis of the anterior part of the larva is simply 

 thrown off, and has no share in forming the epidermis of the adult. 



There are a pair of cephalic imaginal discs and six pairs of 

 thoracic discs. Two pairs, a dorsal and a ventral, give rise to each 

 thoracic ring, and the appendages attached to it. 



Though, as mentioned above, no evidence has yet been produced 

 to shew that the imaginal discs, of Musca are derived from the em- 

 bryonic epiblast, yet their mode of growth and eventual fate proves 

 beyond the shadow of a doubt that they are homologous with the 

 imaginal discs of Corethra. Their earliest origin is well worth further 

 investigation. 



The metamorphosis of the internal organs is still more striking 

 than that of the external. There is a disruption, total or partial, of 

 all the internal organs except the generative organs. In the case of 

 the alimentary tract, the Malpighian vessels, the heart and the 

 central nervous system, the disruption is of a partial kiud, which has 

 been called by Weismann histolysis. The cells of these organs undergo 

 a fatty degeneration, the nuclei alone in some cases remaining. The 

 kind of plasma resulting from this degeneration retains the shape 

 of the organs, and finally becomes built up again into the correspond- 

 ing organs of the imago. The tracheae, muscles and peripheral nerves, 

 and an anterior part of the alimentary tract, are entirely disrupted. 

 They seem to be formed again from granular cells derived from 

 the enormous fat body. 



The phenomena of the development of the Muscidte are undoubtedly of 

 rather a surprising character. Leaving for the moment the question of 

 the origin of the pupa stage to which I return below, it will be admitted 

 011 all hands that during the pupa stage the larva undergoes a series of 

 changes which, had they taken place by slow degrees, would have involved, 

 in such a case as Musca, a complete though gradual renewal of the tissues. 

 Such being the case, the cells of the organs common to the larva and the 

 imago would, in the natural course of things, not be the same cells as those 

 of the larva but descendants of them. We might therefore expect to find 

 in the rapid conversion of the larval organs into those of the adult some con- 



