THE DEVELOPMENT OF THE NYMPH. 317 



the same relative positions as the parts subsequently developed 

 from them ; it is only the paraderm which is reversed by the 

 invagination, its thoracic extremity beinp^ in front of its cephalic 

 portion. As the rudimentary nerves and tracheae of the nymph 

 are all attached to the discs, no disturbance of their relations 

 occurs during the process of evagination, and the reversal of 

 the parablast, which is attended by its contraction, takes place 

 without any disturbance of the internal organs. 



The Evagination of the thorax, and afterwards that of the 

 head, is undoubtedly effected by the contraction of the ab- 

 dominal paraderm, which forces the pseudo-yelk forwards into 

 the thoracic and cephalic cavities. As has been already 

 observed, Reaumur came to the conclusion that the process is 

 a mechanical one, and Weismann made the following remarks 

 on the evolution of the head [2, p. 173] : 



' On the fourth day of the pupa stage the head is pushed 

 forwards out of the hollow in the thorax in which the discs are 

 developed ; it then unites with the front of the thoracic integu- 

 ment. What the nature of the force is which produces the 

 forward movement of the head may be subject to discussion, 

 but I can assert with confidence that it is not the result of 

 growth, but is purely mechanical. I once found the thorax 

 well developed in a fourth-day pupa, but no head was visible ; 

 the latter appeared after the preparation had lain for som.e 

 hours under the pressure of the cover-glass, although the 

 nymph was of course dead.' 



This is merely a repetition of Reaumur's experiment under 

 somewhat different conditions. As Weismann, however, 

 makes no reference to Reaumur, I think it evident that 

 he was not at that time conversant with his memoirs. 

 Reaumur, Weismann, and more recently Van Rees, have 

 ascribed the evolution of the head and thorax to the con- 

 traction of the larval muscles before their final degeneration. 



I think there is little doubt that the last muscular contraction 

 gives rise to the shortening of the pronymph, by which it is 

 drawn back in the pupa-case. This retraction releases the 

 pharyngeal skeleton of the larva, which is subsequently found 



