BETINA OE THE BLOWELY. 
417 
represented are to be considered as nerves ? No one lias yet 
figured one satisfactory representation of the optic-nerve fibres 
entering the great rods. Dr. Hickson says, “ Morphology teaches 
us that the great rods are nerve- terminals.” To appeal to mor- 
phology to settle the question appears to me to show on how 
slender a basis of observation the received view rests, and I should 
myself regard an appeal to morphology as one which is fatal to 
the received view ; for, if morphology teaches us anything on this 
subject, it is that the retinal end-organs belong to that part of 
the epiblast from which the great nerve-centres are developed, 
and that the dioptric structures arise from the superficial or 
cutaneous epiblast. 
DESCRIPTION OF PLATE XXVII. 
Fig. 1. A section of the retina of a Hawk-moth ; partly drawn from a photograph 
and finished from the section, gt, great rods ; m, basilar membrane ; 
b, bacillary layer ; n, optic nerve ; tr, tracheal vessel. 
2. A section of the retina of a Blowfly, c, chaplet-cells of Viallanes. 
i^-inch objective, water-immersion. 
3. A portion of the retina of a Hawk-moth ; drawn from a photograph, 
with details added from the specimen. The tracheal vessels seen 
passing through the basilar membrane are much more distinct in the 
photograph than in the specimen seen by the microscope ; these are 
represented in the drawing as they appear in the photograph. 
4. A transverse section through the bacillary layer of the retina of a Blow- 
fly which had just emerged from the pupa. 
5. A section of the optic disc and cephalic ganglion of a 3-day-old pupa. 
o, optic disc; st, stalk; rt, retina. 1-inch objective. 
6. A portion of the same, showing the retina and inner extremity of the 
stalk. 
7. A portion of the optic disc and stalk of the same, oc, optogenic cells ; 
me, mesoblastic cells, f-ineh objective. 
8. A section of the retina of a ten -day-old pupa. Showing the mesoblast 
elements between the retina and the basilar membrane, 4"i nc b 
objective. 
(The letters indicate the same parts in all the figures.) 
