414 
PROF. B. T. LOWNE ON THE 
from tlie 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, whilst 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 opiuiou that the radial stride (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 
