PSYCHE. 



19 



of most of the internal organs is thus 

 not at any time broken. As these pro- 

 cesses go on, the two large head imag- 

 inal discs, which form two irregularly 

 siiaped sacks extending as diverticula 

 from the dorsal wall of the pharynx 

 back to the brain, begin to pass forward 

 dragging the brain with them. Tiieir 

 anterior ends bend and pass \-entrad 

 embracing the pharynx between them. 

 At the same time their communications 

 with the pharynx enlarge and their 

 lumena fuse more completely with the 

 pharyngial lumen. These communica- 

 tions now continue to enlarge ; tiiey 

 fuse into one single median opening 

 which, ever increasing in size, travels 

 from the anterior end of the pharynx 

 posteriad, obliterating the dorsal phar- 

 yngial wall in its course. Finally the 

 lumena of the discs and that of the 

 pharynx become completely merged 

 and form together a single contintious 

 space, and the walls of the discs and of 

 the pharynx form a single continuous 

 vesicle. This is the head-vesicle of 

 Weismann * and Van Rees j which is 

 destined to become the imaginal head. 

 On its anterior ventral surface epithelial 

 thickenings appear which are destined to 

 form the imaginal antennae and mouth 

 parts, while at its posterior end are 

 thickenings which are to become the 

 compound eyes and which are still in 

 contact with the brain. The head- 

 vesicle remains buried within the pupal 

 thorax until near the end of the pupal 



period when it evaginatcs and forms the 

 imaginal head. This evagination has 

 been observed by Weismann -1: to be the 

 consequence of the pressure of blood 

 whicli at the right moment rushes from 

 the abdomen into the thorax and pushes 

 the head- vesicle forward. 



The metamorphosis of tlie thorax goes 

 on simultaneously with the formation of 

 the head-vesicle. In proportion as the 

 larval hypodermis disappears under the 

 attacks of the phagocytes, as I have 

 alread}' mentioned, the edges of the 

 imaginal discs grow and take its place, 

 forming the imaginal hypodermis. 

 As we have seen, there are six pairs of 

 these discs, three dursal and three 

 ventral, and they are in the center of 

 the larva. Each disc is, however, con- 

 necteil with that portion of the hypo- 

 dermis of the segment to which it 

 genetically belongs and where it is des- 

 tined to appear as an extremity, by a 

 very fine, hollow chord. This chord, 

 now, begins to shorten and its lumen to 

 enlarge. The disc is thus brought 

 nearer the surface and, as it advances, it 

 increases in size. The lumen of the 

 chord then opens through the hypoder- 

 mis to the outside, and finally becomes 

 so wide and the chord itself so short 

 that the disc is brought through the 

 hypodermis to the outside. The hol- 

 low chord has of course been obliterated 

 by this process and the edges of the 

 proximal end of the disc brought into 

 direct connection with the hypodermis. 

 The disc has by this time assumed its 



