FORMATION OF THE PRONYMPH FROM THE LARVA. 313 



that a very thick layer of parablast surrounds the whole tube 

 (PI. XIX., />). The contraction of this layer is apparently the 

 cause of the shortening of the gut, and probably not only 

 causes the epithelium to accumulate in a mass, but also in- 

 vaginates the muscular coat of that portion which has no 

 imaginal cells between it and the epithelium, or, in other 

 words, draws the degenerating and contracted intestine into 

 the interior of the new layer of imaginal cells. The similarity 

 of the fusiform cells to the muscular elements of the intestinal 

 wall is certainly very striking. 



The further development of the new mesenteron is one of 

 the most extraordinary phenomena which occur in the pupa 

 state. As it does not take place until the fourth or fifth day, it 

 will be described in the next section. 



h. The Position of the Neuroblast and the Development of the Stomo- 

 daeum of the Pronymph. 



The description given by Kowalevski of the manner in which 

 the great crop of the larva disappears differs entirely from that of 

 Van Rees. The Russian naturalist believed that it degenerates 

 in situ. The statements made above agree more closely with 

 those of Van Rees. Kowalevski's error originated by his not 

 having observed the remarkable change which takes place in 

 the position of the neuroblast at this period. The axis of the 

 central nervous system, which in the larva is parallel with 

 that of the body, turns through a right angle on the first 

 day of the pupa stage, so that it is transverse to the long axis 

 of the pupa, and transverse sections are no longer across, but 

 parallel to the axis of the nervous system, the dorsal surface 

 of which looks backwards, and the posterior extremity of which 

 is ventral. At the same time there is a great increase in the 

 size of the infra-cesophageal ganglia, which now lie in front 

 of, and not below, the supra-oesophageal nerve centres 

 (Pi. XIX., 5, 0- 



The crop, when it is withdrawn into the oesophagus, forms a 

 swelling which lies in front of and above the infra-cesophageal 

 ganglia, which Kowalevski mistakes for the hemispheres, or 



21 — 2 



