THE LMAGINAL DISCS. 



Si 



the compound eye (the Dioptron, Mihi), whilst the retinal 

 end organs originate from the retinal disc (Fig. 13, rt). This, 

 which I have not included amongst the head discs, is formed 



FIG. 14. Transverse Sections through the head discs (see Fig. 13): d v, dorsa 

 vessel ; ;-, ring ; /, provisional sac ; o, optic ; a, antennal ; /, leg ; w'd, wing 

 discs ; a; oesophagus ; s, lingual gland ; pc d, prefacial rudiment ; f, fat'cell ; 

 tt; trachea ; sp, suspensory membrane ; cp, cephalo-pharyngeal band. 



on the outer surface of the neuroblast, and is entirely enclosed 



in its capsule. In its relation with the nerve-centre it presents 



a striking analogy with the primary optic vesicle of a vertebrate. 



The Optic Stalk (PI. III., o st, PL IV., and Fig. 13, o s) 



6 



