414 



PROF. B. T. LOWNE ON THE 



from the hemisphere, the interspace being filled with the granular 

 yelk-like substance of the somatic cavity of the pupa. The 

 whole dioptron is developed by a division of the optogenic cells, 

 as Claparede long ago showed. Each original cell corresponds 

 to a single corneal facet. These cells form almost hemispherical 

 projections on the outer surface of the disc and are soon covered 

 by an extremely thin cuticular layer. 



The cuticular layer is seen in my sections to dip slightly 

 between the cells, w^hilst the corneal lens is secreted subsequently 

 between the cell and the primitive cuticular layer. The lenses 

 are, as I have already described them, perfectly distinct from the 

 chitinous layer, giving rise to the condition I have designated 

 the kistoid cornea. In adult pupae the distinction is perfectly 

 apparent, although Dr. Hickson has denied that my description 

 is correct ; the most patient reinvestigation entirely confirms 

 my former statement. 



So far my investigations entirely accord with Weismann's de- 

 scription. Weismann, however, believes that the great rods 

 contain a nervous structure, which he describes, from optical 

 sections, as resembling a bundle of fine, highly refractive, con- 

 ducting threads ending at the crystalline cone. He has nothing 

 to say of their manner of development, and only expresses the 

 opinion that they appear more like definite threads than the angles 

 of a solid rod. 



These so-called axial threads, as I have stated above, are well 

 seen in numerous transverse sections to be mere folds of a 

 chitinous membrane enclosing a considerable empty cavity. 



Weismann's description of the development of the nervous 

 structures is as follows : — " The thin nerve-cord (Stiel) which 

 unites the optic disc to the hemisphere still appears on the fifth 

 day as a nervous cord ; but on the twelfth day the pedicle can 

 no longer be seen." He concludes, however, that it has spread 

 out into an invisible layer over the whole surface of the gan- 

 glion. That he should have arrived at such a conclusion without 

 sufficient evidence is quite unlike him. If, as he says and as is 

 certainly the case, the nerve disappears entirely between the 

 fifth and twelfth day, the opinion that the radial striae (which, he 

 says, appear later between the disc and the hemisphere) are the 

 same nerve spread out, is not founded on fact. 



We must remember that Weismann regarded the discs as 



