146 THE THIRD DAY. [CHAP. 



and cones are formed as prolongations or cuticularizations of the 

 cells which eventually form the outer granular layer. The layer 

 of cells external to the molecular layer is not divided till 

 comparatively late into the inner and outer granular (nuclear) 

 layers, and the interposed outer molecular layer. 



Lowe' has recently written an elaborate paper on this subject 

 in which he arrives at very different results from KoUiker 

 and other observers. 



According to him only the outer limbs of the rods and 

 cones, which he holds to be metamorphosed cells, correspond to 

 the epithelial layer of the brain. 



The changes described above are confined to that 

 portion of the retina which lies behind the ora serrata. 

 In front of this both walls of the cup coalesce as we 

 have said into a cellular layer in which a deposit of 

 pigment takes place. 



At a very early period a membrane appears on the side of 

 the retina adjoining the vitreous humour. This membrane is 

 the hyaloid membrane. It is formed at a time when there is no 

 trace of mesoblastic structures in the cavity of the vitreous 

 humour, and must therefore be regarded as a cutioular deposit 

 of the cells of the optic cup. 



The optic nerve. The optic nerves are derived, 

 as we have said, from the at first hollow stalks of the 

 optic vesicles. Their cavities gradually become oblite- 

 rated by a thickening of the walls, the obliteration 

 proceeding from the retinal end inwards towards the 

 brain. "While the proximal ends of the optic stalks 

 are still hollow, the rudiments of the optic chiasma 

 are formed at the roots of the stalks, the fibres of 

 the one stalk growing over into the attachment of the 

 other. The decussation of the fibres would appear 

 ' Archiv fur mikr. Anat. Vol. xv. 



