416 



PROF. B. T. LOWNE ON THE 



sections to mistake fibres running between for fibres entering tlie 

 optogenic cells. 



The nerve-papilla, from which the optic ganglia, the optic nerve, 

 and the retina are developed, gradually grows outwards towards 

 the dioptrou (Plate XXYII. figs. 5-8, n). It is at first covered 

 by a layer of columnar cells , which represent the epiblast of the 

 nerve-centre ; from this layer the bacillary layer of the retina is 

 developed. These cells become converted into the retinal end- 

 organs. The mesoblastic spongy tissue is gradually absorbed or 

 converted into tracheal and connective elements, which ultimately 

 form a thin layer between the retina and the basilar membrane 

 of the dioptron. 



The retina, even when the insect is nearly ready to escape from 

 the pupa, is still separated from the dioptron by a space filled 

 with branching cells (Plate XXVII. fig. 8, mc) and secondary yelk, 

 so that the supposed entrance of nerve-fibres into the dioptron 

 cannot be explained by any known process of development. 



The continuity of the tracheae of the dio^Jtron and those of the 

 mesoblast is the result of the penetration of the Litter between 

 the great rods during their inward growth ; but during this period 

 the nervous papilla is separated by a wide space filled with 

 secondary yelk and reticular mesoblast from the ingrowing epi- 

 thelial structures of the dioptron. 



Thus, if my observations are correct, the retina, like that of a 

 vertebrate, is entirely formed as an outgrowth from the central 

 nervous system, while the dioptron, like the crystalline lens 

 and the refractive structures generally, is formed from the ex- 

 ternal epiblast, which is more or less invaded by mesoblastic 

 elements. With regard to the retina itself, it is undoubtedly, 

 like the nerve-centres, no less epiblastic in the insect than in the 

 vertebrate, as the hemispheres themselves, as well as the ventral 

 ganglia, are formed from the embryonic epiblast. 



In conclusion, I would add that it is scarcely fair to expect me 

 to prove a negative, i. e. that no nerve-fibres pass to the dioptrou. 

 The onus rather lies with my opponents to prove that the great 

 optic nerve does enter the dioptron, and to find its terminals. 

 Even the most cursory glance at the works of Dr. Hickson, M. 

 Berger, M. Viallanes, and others will show that they have given 

 totally dissimilar representations ; of these Dr. Hickson's are 

 correct enough as representations of tracheal and mesoblastic 

 skeletal tissues. I would ask, Which of the various structures 



