RETTNA or THE BLOWFLY. 



417 



represented are to be considered as nerves ? N"o one has yet 

 figured one satisfactory representation of the optic-nerve fibres 

 entering the great rods. Dr. Hicksou says, " Morphology teaches 

 us that the great rods are nerve-terminals." To appeal to mor- 

 phology to settle the questi(m appears to me to sliow 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. 



DESCEIPTION 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 ; 

 6, 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. 



0, 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 ; 



mc, mesoblastic cells. ;^-inch objective. 



8. A section of the retina of a ten-day-old pupa. Showing the mesoblast 



elements between the retina and the basilar membrane. ^-inch 

 objective. 



(The letters indicate the same parts in all the figures.) 



