82 THE LARVA OF THE BLOW-FLY. 



consists of a thick, hollow, neural sheath of stellate mesoblastic 

 cells, enclosing a few nerve fibres. It arises from the upper and 

 outer part of the hemisphere in the young larva, but as develop- 

 ment progresses, owing to the great enlargement of the hemi- 

 sphere, it is gradually pushed downwards and outwards, so that 

 it finally arises from the lower and posterior portion. The 

 nerve-fibres of the optic stalk traverse the mesoblast of 

 the optic disc, and form a small nerve, which passes forward 

 by the side of the pharynx and oesophagus, and probably 

 supplies the simple eyes at the extremity of the maxilla. I 

 cannot be certain of this, however, as I have been imable to 

 trace the entire course of this nerve. 



Each lateral half of the frontal region is developed from the 

 epiblast of the posterior part of the inner wall of the disc sac, 

 which is connected with the lower border of the optic disc 

 behind, and supports the antennal rudiment in front. The 

 antenna originates from a central papilla, surrounded by two 

 concentric rings. The central papilla becomes the third 

 antennal joint. In front of the antenna a thin band of epi- 

 blast is continued forwards ; it terminates in a bulb-like 

 prefacial enlargement, and sends a process into the cephalo- 

 pharynx, between the cornu and the inferior process, the 

 rudiment of one lateral half of the fulcrum of the imago. 



Menzbier* and Kiinckel d'Herculais [25] regarded the optic 

 discs and the antennal and prefacial rudiments as distinct, and 

 have deduced theoretical conclusions on the segmentation of 

 the head from this assumption. There is a distinct continuity 

 of the epiblast, and the whole are enclosed in a single disc sac. 

 I am unable, therefore, to admit the validity of such conclusions. 



The Appendicular Discs of the Head. The maxillary discs (Fig. 13, 

 mx, and Fig. 8, 2) first appear late in larval life as invaginations 

 of the hypoderm of the stomal disc. One appears close to the 

 outside of the attachment of the great hook on each side. The 

 labial discs are represented in the adult larva by a small group of 

 cells on either side of the lingual (salivary) duct near its orifice. 



* ' Ober das Kopfskelet u. s. der ZvveiflUgler.' Bull. Soc. Imp. Nat., Moscou, 

 torn. Iv., 1880. 



