314 rHE DEVELOPMENT OF THE NYMPH. 



supra-oesopha^eal nerve centres. This change of position in 

 the nerve centres makes it appear as if the crop still remains 

 dorsal to the hemispheres in transverse sections until the real 

 nature of the nerve ganglia upon which it lies is recognised; for 

 the infra-cesophageal nerve centres, without careful examina- 

 tion in their new position and greatly increased development, 

 are readily mistaken for the hemispheres. Van Rees, without 

 remarking the changed position of the neuroblast, observed 

 the withdrawal of the crop towards and afterwards into the 

 oesophagus, whilst Kowalevski drew his conclusions entirely 

 from an erroneous interpretation of the appearances presented 

 by transverse sections. 



Kowalevski derives the new stomodaeum from the proven- 

 tricular ring. Van Rees from a group of cells which appear at 

 the point of invagination of the crop. My own view is that the 

 remains of the larval stomodaeum become invested by a layer 

 of parablast ; and that the new stomodaeum is developed mainly 

 from the embryonic cells which form part of the head discs, 

 from the lingual discs which are seen on either side of the labium 

 at the orihce of the salivary duct, and probably from the imaginal 

 rudiments in the larval oesophagus, described by Van Rees, 

 which I have not been fortunate enough to identify. These 

 may possibly form the new crop, at first a short diverticulum 

 of the oesophagus corresponding with the origin of that of the 

 larva, which, it will be remembered, is a ventral diverticulum 

 of the oesophagus, and not a dorsal one, as is generally 

 believed. 



I have already alluded to the discontinuity of the stomodaium 

 and proventriculus of the pronymph. My sections of pupae of 

 the second day show no trace of the oesophagus immediately in 

 front of the proventricular ring, and still further forward one 

 comes suddenly upon the blind end of the degenerating 

 oesophagus. This is surrounded by numerous parablast 

 cells. 



I strongly suspect that the blind posterior end of the oeso- 

 phagus is connected with the proventriculus by a solid cord of 

 parablast during the whole metamorphosis, although at this 



