326 THE DEVELOPMENT OF THE NYMPH. 



makes its appearance, dividing each into a proximal and a 

 distal portion. The external or distal portion is larger, and 

 subsequently develops the great optic ganglion and the retina 

 of the compound eye ; the smaller or proximal inner part be- 

 comes the supra-oesophageal nerve centre. 



I have already referred to a remarkable epithelial disc, the 

 retinal imaginal disc (p. 70) as it occurs in the larva (Fig. 13, rt\ 

 which has not been seen by any previous observer. A com- 

 parison of this disc with the cerebral vesicle of the embryo 

 (Fig- 44) renders it probable that the disc cavity is the remains 

 of the vesicle or part of the vesicle itself. Moreover, the disc 

 cavity of the retinal disc is prolonged as a central cavity through 

 the optic stalk (see Fig. 13 and PL IV.). In the pupa on the 

 third day this disc has greatly increased in size, and its rela- 

 tions are represented in Fig. 46. In this stage it forms an 

 optic cup, the inner surface of which ultimately becomes applied 

 to the inner surface of the optic disc (op d), and forms the 

 retina of the compound eye. This disc and its stalk, the optic 

 tract, form the greater part of the external portion of the de- 

 veloping hemisphere. Sections through the optic cup and tract 

 give very various and perplexing appearances, and it is only by 

 a fortunate series that I was able to interpret its true structure. 

 It will be at once apparent that tangental sections exhibit a 

 closed sac, and, as the epithelial disc rt is folded and corru- 

 gated, some are not readily understood. Again, it is impossible 

 to get a single section which shows the canal in the optic stalk 

 and the retinal disc, as the optic tract is curved. It is not so 

 difficult, however, to demonstrate the further evolution of the 

 retina from the retinal disc. 



As my own views on the structure and morphology of the 

 compound eye are greatly at variance with those which are 

 generally received, I must defer the further consideration of 

 this subject until I give the full details of my investigations. 

 In the meantime I would refer my readers to my published 

 papers on the subject.* 



Lowne, B. T., 'On the Compound Vision and the Morphology of the Eye 

 in Insects,' Trans. Linn. Soc., 2nd series, Zool., vol. ii., 1884. ' On the Struc- 

 ture of the Retina of the Blow-fly,' Journ. Linn. Soc. Zool., vol. xx., 1889. 



