THE DEVELOPMENT OF THE NYMPH. 329 



imaginal cells.' I am at a loss to reconcile this statement with 

 the general tenor of Ganin's work. 



• Viallanes supposes that the larval hypoderm is destroyed 

 before the discs enclose the pseudo-yelk. In this I agree with 

 him, but only in a certain sense. Viallanes states that the 

 pseudo-yelk is only covered by a fine cuticle before the discs' 

 grow over it. In this he was certainly wrong. Kowalevski, on 

 the other hand, found that the discs grow over a cellular layer, 

 which is only removed from the central area of the disc as its 

 growth advances, and that they always overlap this layer by 

 their edges. This view is also supported by Van Rees, but 

 both authors regard it as the hypodermis of the larva. It is my 

 paraderm. 



The Imaginal Discs of the Abdomen (PI. XVIII., Fig. 5, d) are 

 two in number on each side of each segment, except the last, 

 which only has a single pair of discs visible externally. The 

 ventral pair lie one on each side of the anus, and are in- 

 vaginated like the thoracic discs. This fact is noted b}' 

 Kowalevski. 



The position of the discs is well seen in PI. XX., Fig. 4, which 

 represents a very beautiful preparation made in the following 

 manner. The pronymph was removed from the pupa-case, after 

 heat-coagulation had been effected by boiling in water for a 

 few seconds ; it was immersed for three hours in a o'l per 

 cent, solution of hydrochloric acid, transferred to a mixture of 

 glycerine and water, gradually strengthened until it contained 

 50 per cent, of glycerine, and afterwards stained in borax 

 carmine to which an equal volume of glycerine had been added. 

 By this treatment the imaginal discs are the only part of the 

 integument which receives the stain, and their limits are beauti- 

 fully seen. 



My specimen also exhibited a third and a larger more 

 faintly stained region on each side of each segment, midway 

 between the dorsal and ventral disc. These areas correspond 

 with the large subcutaneous cells, which W^ielowiejski termed 

 oinocytes (see p. 276). 



Van Rees believes that each abdominal segment has a third 



22 — 2 



h 



