552 THE SEXSES AND SKXSORY OXGAXS. 



\Yhatever 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 : 



1)1 ,i KUTION OK 1'IATK \.\XI.\. 



A section through a portion of the dinptron and retina of a Blow-fly pupa, three or 

 four days before the end of the pupa stage. /', basilar membrane ; ,; r , .;', granule 

 cells in the sub-dioptric space ; />, lar^e mesol>la>tic cells ; p >; pre-retinal lamella ; 

 rf, retina. 



