So THE LARVA OF THE BLOW- FLY. 



lobes ; and that the ring is an indication of its origin from 

 two lateral halves. The band and ring consist in the adult 

 larva of a solid epithelial tissue, in which the intercellular 

 substance is much developed ; it is exceedingly like cartilage, 

 and closely resembles the endo-thoracic skeleton of the scorpion 

 described by Prof. R. Lankester.* The changes which the 

 ring undergoes in the later stages of the larva are very marked ; 

 in the feeding larva it is nearly vertical, and the nervous elements 

 on its exterior are very distinct ; in the resting larva it grows 

 rapidly and becomes nearly horizontal, so that its upper border 

 becomes anterior and its lower border posterior. The nerve 

 elements are then inconspicuous in relation to the skeletal 

 quasi-cartilaginous substance of the ring itself. 



The ring and cephalo-pharyngeal band support the great 

 cephalic disc sacs, the provisional membranes of which are 

 continuous with its edges (Fig. 14, s/>). 



The disc sac is differentiated into the optic, antennal, pre- 

 facial and pharyngeal rudiments (Fig. 13). 



The Optic Rudiments or Discs are cup-shaped in the feed- 

 ing larva, and lie in front of and above the hemispheres, with 

 which they are connected by the optic stalks. As development 

 progresses, their outer and inferior borders become greatly 

 thickened and folded, so that they are sub-triangular when 

 seen in profile (Plate IV.). 



Weismann compared the optic disc to a mushroom with an 

 excentric stalk, the optic stalk. The latter is solid at first, but 

 a canal is afterwards formed in its interior, through which 

 the retinal disc ultimately reaches the inner mesoblastic surface 

 of the optic disc. I shall show hereafter that the optic disc 

 is concerned in the development of the dioptric structures of 



* ' On the skeleto-trophic tissues and coxal glands of Limulus, Scorpio and 

 Mygale.' Quart. Journ. Micros. Sc, vol. xxiv., new series, 1884. 



Plate IV.— The Neuroblast and the Imaginal Discs co.nnecteu with it 



IN THE Resting Larva. 



r. The ring ; d v, dorsal vessel ; a; the oesophagus ; o/> d, optic disc ; an d, antennal 



disc ; pr d, prefacial riuliincnt and pharyngeal disc ; p/i, cephalo-pharynx ; /r, trachea ; 



« ,/, prothoracic leg disc ; m/ d, niesotlioracic leg discs ; n, nerve. 



