424 1NSECTA. 



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 kind, which has been called by Weismann 

 histolysis. The cells of these organs undergo a fatty degenera- 

 tion, 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 Muscidae 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 on 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 condensa- 

 tion, so to speak, of the process of ordinary cell division. Such condensations 

 are probably represented in the histolysis in the case of the internal organs, 

 and in the formation of imaginal discs in the case of the external ones, and 

 I think it probable that further investigation will shew that the imaginal 

 discs of the Muscidae are derivatives of the embryonic epiblast. The above 

 considerations by no means explain the whole of Weismann's interesting 

 observations, but an explanation is I believe to be found by following up 

 these lines. 



More or less parallel phenomena to those in Insects are found in the 

 development of the Platyelminthes and Echinoderms. The four disc-like 

 invaginations of the skin in many larval Nemertines (vide p. 198), which 

 give rise to the permanent body wall of the Nemertine, may be compared to 

 the imaginal discs. The subsequent throwing off of the skin of Pilidium or 

 larva of Desor is a phenomenon like the absorption of part of the larval 

 skin of Musca. The formation of an independent skin within the first larval 



