THE IMAGINAL DISCS. 



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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 i\ dorsa 

 vessel ; r, ring; /, provisional sac ; 0, optic ; a, antennal ; /, leg; ica', wing 

 discs ; a-, oesophagus ; s, lingual gland ; pc d, prefacial rudiment ; / fat cell ; 

 ti; 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., si, PI. IV., and Fig. 13, s) 



6 



