HOW THE WIXGS GET OUTSIDE OF THE BODY 133 



says Gonin, we do not see the why or the how. The partition of 

 the sac has disappeared, and with it the cavity and the leaf of the 

 envelope. 



It appears probable that the partition has been destroyed, because the space 

 between the two teguments is strewn with numerous bits, many of which adhere 

 to the chitinous integument, while others are scattered along the edges of the 

 wings, in their folds, or between the wings and the wall of the thorax. 



Another series of sections showed that the exit of the fore wings had been 

 acco'mplished, while the hinder pair was undergoing the process of eversion. 

 In this case the partition showed signs of degeneration : deformation of the 

 nuclei, indistinct cellular limits, pigmentation, granular leucocytes, and fatty 

 globules. 



After the destruction of the partition, what remains of the layer of the 

 envelope is destined to make a part of the thoracic wall and undergoes for this 

 purpose a superficial desquamation. The layer of flattened cells is removed and 

 replaced by a firmer epithelium like that covering the other regions. It is this 

 renewed hypodermis which conceals the wing within, serves to separate it from 

 the cavity of the body, and gives the illusion of a complete change in its situa- 

 tion. Other changes occur, all forming a complete regeneration, but which does 

 not accord with the description of Van Rees for the Muscidse. Finally, Gonin 

 concludes that the de'bris scattered about the wing comes from the two layers of 

 the partition of the sac, from the flattened hypodermis of the renewed envelope, 

 from the chitinous cuticle of the wing, and from the inner surface of the 

 chitinous integument. 



He thinks that the metamorphosis of Pieris is intermediate between the two 

 types of Corethra and of Musca, established by Weismann, as follows: 



Corethra. The wing is formed in a simple depression of the hypodermic 

 wall. No destruction. 



Pieris. The rudiment is concealed in a sac attached to the hypodermis by a 

 short pedicel. Destruction of the partition and its replacement by a part of the 

 thoracic wall by means of the imaginal epithelium. 



Musca. The pedicel is represented by a cord of variable length, whose cavity 

 may be obliterated (Van Rees). The imaginal hypodermis is substituted for 

 the larval hypodermis, which has completely disappeared, either by desquama- 

 tion (Viallanes), or by histolytic resorption (Van Rees). 



Extension of the wing ; drawing out of the tracheoles. When it is 

 disengaged from the cavity, the wing greatly elongates and the 

 creases on its surface are smoothed out ; the blood penetrates between 

 the two walls, and the cellular fibres, before relaxed and sinuous, 

 are now firmly extended. 



Of the two trachea! systems, the large branches are sinuous, and 

 they are rendered more distinct by the presence of a spiral mem- 

 brane ; but the two tunics are not separated as in the other tracheae 

 of the thorax ; moreover, the mouth choked up with debris does not 

 yet communicate with that of the principal trunk. The bundles of 

 tracheoles on their part form straight lines, as if the folds of the 

 organ had had no influence on them. As they have remained bound 

 together, apart from the chitinous membrane of the tracheal trunk, 



