552 THE SENSES AND SENSORY ORGANS. 
Whatever the origin of the cells of the pseudo-cone, I think 
they are distinct from the sub-corneal cells, but they appear to 
correspond morphologically with the cells from which a true 
cone is developed in the euconic eye, and I am doubtful if this 
is a mesoblastic structure. It is possible that the cells of the 
pseudo-cone originate from epiblast, and it is difficult to say 
at what period they first appear, as they lie amongst the cells 
which subsequently become pigmented, and form the outer 
sheath of the chamber and the outer iris cells. 
Development of the Basilar Membrane.—Although in the adult 
imago in the Blow-fly and most other insects the basilar mem- 
brane is represented by a thin and apparently chitinous trans- 
parent membrane with a layer of cells on either surface, the 
manner in which this membrane is developed indicates that 
it is by no means so simple as it appears in the adult imago. 
All the elements of the pre-retinal space disappear during its 
formation, and the condition of things in the newly-emerged 
imago indicates that it is derived from the compressed con- 
nective-tissue elements of the pre-retinal space, and that the 
pre-retinal tracheal network is really imbedded in the substance 
of this membrane, instead of lying, as it appears to lie, on its 
inner surface. It is, however, possible that the membrane 
is formed from the inner surface of the inner pigment cells, 
which are undoubtedly of epithelial origin, but from its relation 
to the great rods it appears to me more probable that it is 
derived from the cells of the pre-retinal lamella or from the 
intermediate layer of cells in the pre-retinal space, or all three 
layers of cells may be concerned in its formation. 
Development of the Retina.—The origin of the retina from the 
roof of the vesicle of the cerebron which forms the retinal 
disc has already been referred to. The evolution of the in- 
vaginated retinal rudiment apparently takes place in the 
following manner : 
DeEscRIPTION OF PLATE XXXIX. 
A section through a portion of the dioptron and retina of a Blow-fly pupa, three or 
four days before the end of the pupa stage. 4, basilar membrane ; y, g, granule 
cells in the sub-dioptric space ; /, large mesoblastic cells ; p 7, pre-retinal lamella ; 
rt, retina. 
