70 THE LARVA OF THE BLOW- FLY. 



ganglia ; indeed, towards the posterior extremity of the organ 

 they appear to replace the embryonic elements entirely. 



Some of these cells attain very large dimensions, especially in 

 the hemispheres close to the oesophagus (PI. II., Fig. 3, sc) ; 

 similar cells are also found in the same region in the imago. 



The central substance, Weismann's medulla, consists of non- 

 medullated stroma ; it is divided into two lateral halves, united 

 by a transverse commissure (PL III., Fig. i), and exhibits 

 several distinct tracts. 



In the newly-hatched larva the neuroblast is proportionately 

 much longer than in the adult, and exhibits a distinct division 

 of the pro-cephalic portion of the central stroma into three 

 masses (PI. II., Fig. 7), which correspond with the proto-, deu- 

 tero-, and trito- cerebron of Viallanes, and develop into the 

 cerebron and corpora fungiformia, the corpus centrale, and the 

 olfactory (antennal) ganglia respectively. In the adult larva 

 the ventral portion of the central substance exhibits a large 

 number of distinct bundles of vertical fibres on either side, but 

 contiguous to the middle line. 



The hemispheres are connected with the post-oral or infra - 

 cesophageal portion of the neuroblast by a pair of thick crura 

 (PI. III., in s), and with each other, above by the supra- 

 cesophageal commissure, and below by the conjoined antennal 

 and infra-oesophageal ganglia. 



The rudiments of the corpora fungiformia (cf), and of the 

 cerebron and optic ganglia, of the imago, can be distinctly 

 recognised (d) ; but the most remarkable structure is a layer of 

 infolded epiblast (rt), from which the retina is subsequently 

 formed in the nymph. This is an imaginal disc, which lies 

 within the capsule of the neuroblast, whilst the eye-disc of 

 Weismann, from which the rest of the eye is formed, lies out- 



PLATE III. TRANSVERSE SECTIONS THROUGH THE NEUROBLAST OF THE 



ADULT LARVA. 



FIG. i. Through the Posterior Part ; and FIG. 2. Through the Centre of the 

 Hemispheres, a, l>, and cf, Parts of the corpus fungiforme ; d, the trabecula 

 and its divisions; r r', the ring; rt, retina; c, capsule : s, f, nuclei of infra- 

 cesophageal ganglia ; m s, stroma ; o st, optic stalk ; sf, stalk of inferior meso- 

 thoracic disc ; /, median stomogastric nerve. 



