320 THE DEVELOPMENT OE THE NYMPH. 



The Leg and Wing Discs are first exposed by the retraction 

 of their provisional membranes. Their thoracic edges unite 

 with each other in the median hne of the ventral surface 

 (PI. XX., Fig. i), subsequently from before backwards, and 

 last of all on the dorsum. The appendages are developed 

 as diverticula of the body cavity, which rapidly grow in length 

 and diameter, and assume positions similar to those of the 

 lepidopterous nymph (PI. XX., Fig. 5). 



The dorsal surface of the mesothorax (PI. XX., Fig. 2) is 

 completed by the union of the right and left wing discs. The 

 upper prothoracic discs do not meet until the mesothorax is 

 much enlarged, and the metathoracic discs remain separated in 

 the median line above, or, if they meet at all, only do so by a 

 narrow isthmus. 



The mesothorax, when it is first closed dorsally, is a narrow 

 ring no wider than the mesothoracic segment of the larva, 

 but it increases rapidly from before backwards, and even at 

 first exhibits a depression marking the future position of the 

 post-scutal sulcus (p 166). 



The Head first appears in front of the thorax as a thin, 

 bladder-like projection from its interior. As soon as it is 

 entirely exposed, it is seen to exhibit five convexities, one 

 lateral protuberance on each side corresponding to the optic 

 disc and three median vesicles. The frontal vesicle is most 

 prominent ; it has the facial (antennal) vesicle on its ventral 

 surface, and a narrow rim representing the occipital surface of 

 the head, the posterior vesicle, above and behind it. The thin- 

 walled head consists in great part of the paraderm of the 

 cephalic invagination with the cephalic discs, which form a 

 comparatively small part of its surface ; for some hours after 

 it appears in front of the thorax a great part of it consists 

 of large flattened cells, very distinct from the embryonic epi- 

 thelium of the disc epiblast. 



The Neck. — The constricted neck is developed very slowly by 

 the ingrowth of the parablastic layer, which forms a septum 

 between the head and thorax. Sections through this region, 

 and also those between the segments of the abdomen, exhibit 



