342 THE DEVELOPMENT OF THE NYMPH. 



exist in the nymph ; yet I see no reason to beheve that the 

 dorsal vessel ceases to pulsate, and in the numerous sections 

 which I have examined I can see no evidence of any change in 

 structure. It remains the same as in the larva until the last 

 two or three days of the pupa state, and does not exhibit any 

 traces of degeneration. Kowalevski says that when treated 

 with osmic acid the boundaries of its constituent cells (muscle 

 cells) become more distinct, and that its transverse striations 

 are less marked on the third day of the pupa than in the larva. 

 As, however, the demonstration of the muscle cells of the dorsal 

 vessel by osmic acid is very uncertain at all stages, and the 

 muscular striations vary in distinctness in different prepara- 

 tions, I am not inclined to regard the above statement as 

 important, and Kowalevski leans to the opinion that the dorsal 

 vessel does not undergo histolysis. 



The last-named author erroneously supposed that the dorsal 

 vessel is more deeply seated in the larva than m the pupa ; in 

 the posterior segments of the larva, as already stated, it is 

 placed immediately beneath the integument. This portion is 

 lengthened during the pupa stage, whilst the aortic section, 

 which is deeply seated, becomes shortened. 



Although I believe that the dorsal vessel persists during the 

 whole period of the pupa sleep and performs its function, its 

 form is so changed during the last few hours of this period that 

 the dorsal vessel of the imago cannot be regarded as identical 

 with that of the larva , and I think it probable that all its 

 muscle cells are re-formed from embryonic cells, and gradually 

 replace the muscle cells of the larval heart, since on the tenth 

 or eleventh day of the pupa the dorsal vessel is lined by a 

 double row of musculogenic cells, and from this period its 

 lumen and the thickness of its walls rapidly increase. 



The Ccelom, Cell Chaplets and Pericardial Cells, — The ccelom in 



Description of Plate XXI. 



A median sagittal section of a male nymph from a pupa seven clays old, showing the 

 relations of the nerve centres, dorsal vessel, stomodteum, archenteron, and recto- 

 cloaca! pouch. The coelum is almost entirely occupied by cellular elements 

 derived from the fat bodies of the larva. 



