116 ON THE ANATOMY OP THE PLY. 



Beyond these facts, I can add little with any degree of cer- 

 tainty, except that twenty-four hours after impregnation, the 

 newly developed tracheal vessels of the embryo become suddenly 

 filled with air, and the larva commences to struggle in the egg, 

 but soon ruptures the ovisac and escapes. 



Dr. Weismann has traced the development of the embryo larva far more 

 minutely than I have, in his " Entwicklung Der Dipteren ;" but I confess I 

 do not know how he has attained such well-defined results from the very 

 delicate cellular structures of the embryo, nor do his figures appear to me 

 so accurate as those of the imaginal discs, and the development of the 

 pupa. 



Section XXIII. The Formation of the Pupa. 

 PLATE VI., FIGS. 1 TO 4. 



The nature of the changes which take place during the 

 formation of the pupa have been already briefly indicated. 

 They closely resemble those in the Echinodermata, where 

 the perfect form is developed in the interior of the larva. 

 But the change is even more remarkable in the fly, as 

 all the tissues of the larva undergo degeneration, and the 

 imaginal tissues are re-developed from cells which originate 

 from the disintegrated parts of the larva, under conditions 

 similar to those appertaining to the formation of the embryonic 

 tissues from the yolk. 



The formation of the pupa^ may be conveniently studied 

 under two heads the formation of the pupa skin, and the 

 transformation of the larval tissues. 



The pupa skin is partly formed from seven pairs of delicate 



