CEPHALIC IMAGINAL DISCS 147 



innervated by fine nerves. Althcjugh the iiiiaginal discs of M. do- 

 vtestica are similar in some respects to those of the blow-fly, as 

 described by Lowne, there are several important differences, chief 

 of which is the position of the imaginal discs of the metathoracic 

 legs. 



During the resting period of the larva the cephalic and thoracic 

 discs can be distinguished, but the abdominal discs are small and 

 not so obvious except in sections. 



The cephalic discs. 



The chief cephalic discs are contained in what at first appears 

 to be a pair of cone-shaped structures in front of each of the 

 cerebral lobes of the ganglion (fig. 53, m.c.d.); the cone, however, 

 is not complete. The outer sheath of each of these major cephalic 

 imaginal rudiments is continued dorsally, and joins the cellular 

 structure mentioned previously (see fig. 66), thus enclosing a trian- 

 gular space which is a portion of the ventral sinus. These sheaths 

 are continued anteriorly and are connected to the pharyngeal mass, 

 and it is through this connecting strand of tissue that the discs 

 are everted to form the greater part of the head of the nymph. 

 Immediately in front of the cerebral lobe is the so-called optic 

 disc (fig. 65, o.d.), which in its earlier stages is cup-shaped, but 

 later it assumes a conical form, having a cup-shaped base adjacent 

 to the cerebral lobe. The optic disc is connected to the cerebral 

 lobe laterally by a stalk of tissue, the optic stalk (o.s.), which 

 becomes hollow later, and it is through this stalk that the optic 

 ganglion and associated structures contained in the cerebral lobe 

 appear to evaginate when the final metamorphosis and eversion 

 of the imaginal rudiments takes place. The optic discs form the 

 whole of the lateral regions of the head of the fly. 



The remaining portion of the head-capsule of the fly is formed 

 from two other parts of imaginal rudiments, the antennal and 

 facial discs. The antennal disc (an.d.) lies in front, and internal 

 to, the optic discs. Each consists of an elongate conical structure, 

 in which at a later stage the individual antennal joints can be 

 distinguished. The facial discs {f.d.) are anterior to the antennal 

 discs and extend to the anterior end of the conical structure 



10—2 



