FORMATION OF THE PRO NYMPH FROM THE LARVA. 305 



which the pedicles of the leg-discs are more distinct than in 

 the full-grown maggot. 



As the discs of the wings and halteres are the only ones in 

 which I have not succeeded in tracing any connection with the 

 hypoderm, I am exceedingly gratified to find Van Rees places 

 the connection on the same basis as that of the other discs by 

 direct observation. 



With regard to the function of the pedicles in relation to the 

 evolution of the discs during the formation of the nymph, Van 

 Rees has shown by transverse sections that the provisional 

 cavity opens upon the surface by the shortening and ultimate 

 opening out of the pedicle. His paper is illustrated not only 

 by drawings of the discs at the period of their evolution, 

 but by diagrams. 



d. The Contraction of the Paraderm and Evolution of the Discs. 



In the pronymph stage the imaginal discs all enlarge rapidly; 

 their provisional cavities also become distended with fluid and 

 extend into the pedicles. The latter become shortened, until 

 at length the disc sacs lie in immediate relation with the 

 paraderm. 



A ring of small cells now appears at the junction of the disc 

 sac with the paraderm in the position previously occupied by 

 the pedicle, and the adjacent large flat paraderm cells begin to 

 contract. The area of each cell diminishes and its thickness 

 increases, so that the nuclei of adjacent cells are drawn 

 together. At the same time an orifice appears in the centre of 

 the small cells, which opens into the disc sac, and this rapidly 

 enlarges, so that these sacs are converted into open pouches. 

 The cells of the provisional capsules undergo the same con- 

 traction and thickening as the paraderm cells, a change which 

 brings the disc to the surface. When this is complete, the 

 provisional capsule becomes part of the paraderm. 



The effect of the gradual but continuous contraction of the 

 paraderm has been already alluded to. This contraction occurs 

 chiefly, but not exclusively, in that part which corresponds with 

 the abdomen of the nymph, where the evolution of the disc 



