300 THE DEVELOPMENT OF THE NYMPH. 



tion of the resting larva into a pupa (see p. 3) is accompanied 

 by a separation of the hypoderm from the overlying cuticular 

 layers. The changes which occur in the hypoderm have been 

 variously described, and they are by no means easy to follow. 



Weismann says, speaking of the third-day pupa : ' The 

 thorax of the nymph is already formed by the union of the 

 imaginal discs, but it is not, as one would expect, enclosed 

 within the larval hypodermis. It lies immediately beneath the 

 hornj' pupa-shell. The hypodermis and muscles of the thoracic 

 segments have undergone degeneration and have changed into 

 a fine granular mass, which mixes with the blood in the interior 

 of the developing nymph' [2, p. 165]. Weismann, who be- 

 lieved that the hypodermis of the abdomen of the larva changes 

 directly into that of the imago, gives no nearer details upon the 

 subject. 



In spite of this direct observation of Weismann's, which is 

 perfectly correct, Graber [10, Bd. ii., Figs. 163 and 178] gives 

 a well-known schematic representation, as the result of the 

 observations of Ganin and Viallanes more especially, and 

 represents the Fly-nymph as consisting of a simple abdominal 

 cellular integument, the hypodermis of the larva, with a double 

 thoracic integument, the larval hypoderm, enclosing the newly- 

 developed thorax of the nymph. I may at once observe that 

 this scheme is entirely erroneous. 



The most recent memoir on the Metamorphosis of the 

 Blow-fly is by Van Rees [147], and his observations agree in 

 the main with my own, which only differ in this, that what 

 Van Rees regards as the larval hypoderm I regard as a new 

 formation, developed after the histol3'sis of the larval hypoderm, 

 my paraderm. 



Sections of the resting larva, and of the pupa in its earliest 

 stage before a trace of colour appears in the cuticular shell, 

 exhibit unmistakable histolysis of the hypoderm, the cells of 

 which are invaded by leucocytes at an earlier period than even 

 the larval muscles. All the phenomena described as occurring 

 in the muscles likewise occur in the cells of the larval hypo- 

 derm. The degenerating cells are swollen, their protoplasm 



