FORMATION OF THE PRO NYMPH FROM THE LARVA. 315 



early stage I have been unable to demonstrate the connection. 

 In a later stage the solid parablastic cord uniting the blind end 

 of the new stomodseum with the rudimentary proventriculus of 

 the nymph is sufficiently apparent to leave no doubt in my 

 mind upon the subject. 



The development of the new stomodxum is by no means 

 easy to trace, owing to the rapid changes which occur during 

 the evolution of the head. According to Kowalevski, it is at 

 first a wide, short tube, which only subsequently becomes 

 narrow and elongated. Van Rees failed to observe its first 

 stage, but it is evident that before the evolution of the head it 

 must be a very much shorter tube than when the head appears 

 in front of the thorax. 



2. THE DEVELOPMENT OF THE NYMPH. 



From the end of the second day to the fifth day of the pupa 



stage. 



a. The Position of the Imaginal Discs in the Pronymph. 



The disposition of the imaginal discs in the pronymph is 

 seen in PL XIX., which represents a vertical oeciion in the 

 median plane, from the rectum (r) to the neuroblast (s, t). In 

 front of the neuroblast the section is taken through the discs, 

 about i mm. to the right side of the median plane, except in 

 the case of the wall of the oesophagus and the rudimentary 

 mouth (/, e), which are shown in the median plane. Hence this 

 figure is so far diagrammatic ; it is a construction made 

 from a series of sections, and not a drawing of any one 

 section. 



It will be seen by reference to the plate that the paraJerm, 

 represented by a double outline enclosing a series of short black 

 lines, is invaginated above, and forms the roof of a cavity (/, h}. 

 This I term the ' cephalic involution.' Below and on either 

 side of the cephalic involution there is a crescentic invagination 

 of the paraderm, communicating laterally with the sacs of the 



