FORMATION OF THE PRO NYMPH FROM THE LARVA. 303 



thicker than those of the larval hypoderm, nor are the nuclei 

 larger, as Viallanes states, but smaller. 



Although transverse sections of young pup* show that a 

 considerable space exists between the pupa-shell and the larval 

 hypoderm which is filled by a serous fluid, and which remains 

 for some hours after the paraderm is complete the latter 

 subsequently comes into close relation with the pupa-case, 

 except at its anterior pole. This arises from the shortening of 

 the pronymph, and is the result of the still further invagination 

 of its anterior within its posterior segments. The anterior part 

 of the abdominal wall, or, rather, of that part of the paraderm 

 which has the abdominal discs attached to it, now forms a 

 double fold, enclosing the parts derived from the thoracic 

 segments. I term this the ' antericr abdominal fold.' 



The anterior abdominal fold is deepest on the ventral aspect 

 of the pronymph, and is probably produced by the last con- 

 tractions of the longitudinal ventral muscles of the larva. 

 Subsequently the whole paraderm undergoes contraction with 

 important results ; for I regard this contraction as one of the 

 essential factors in the evolution of the discs. Van Rees 

 ascribes all the contractions which give rise to the definitive 

 form of the body of the nymph to the larval muscles ; yet 

 these contractions occur at a time when, if any larval muscle 

 remains, it is entirely detached from the integument of the 

 pronymph. 



Kowalevski actually observed slow but rhythmic contractions 

 of the body of the pronymph after the whole of the larval 

 muscles have been disintegrated by histolysis. I have no doubt 

 whatever of the contractile power of the paraderm. 



When the paraderm is completely developed, the thoracic 

 portion of the pronymph is entirely covered by the anterior 

 abdominal fold, whilst the cephalic portion is similarly in- 

 vaginated within the thoracic. 



c. The Relation of the Imaginal Discs to the Paraderm. 



I have already drawn attention to the true morphological 

 character of the imaginal discs (p. 73). At the time I wrote 



