192 MR. H. GRAY ON, THE DEVELOPMENT OF THE RETINA AND OPTIC NERVE, 
bounded by a dark outer line ; this is now the capsule of the lens, the inner one the 
bounding margin of the lens itself. Nor could I ever see satisfactorily any doiibling- 
in of the retina so as to form two layers, for in no position that I put the embryo, in 
each of the several examinations that I made, could I ever detect but a single layer ; 
and besides, as I shall show hereafter, Jacob’s membrane is not developed until a 
much later period. I cannot forbear here adding the conclusions Bischoff has come 
to from his observations on this point. Although many anatomists of great reputa- 
tion follow the opinion of Huschke, he says, “ Malgre ces autorites je dois avouer 
qu’il m’a 6te impossible meme chez de tres jeunes embryons de chien, de lapin, et de 
rat, d’aperqevoir sur la face anterieure de I’oeil aucune trace d’une semblable intro- 
mission des teguments exterieurs, quoique dans certains cas, je sois rest4 incertain de 
savoir s’il existait deja une capsule cristalline et un cristallin*.” 
At the seventieth hour (fig. 7) the circular outline of the eyeball has become quite 
distinct, not only externally, but now the line may be traced internally, where pre- 
viously it was only indicated by the dark circular granular mass, the forming eyeball. 
The lens is also more distinctly seen bounded by its well-defined circular border, in- 
dicating its forming capsule, and the nucleus in its centre is plainly visible. When 
examined on its under surface, the embryo lying on its dorsal aspect, the original 
protrusion is seen to have become still more pyriform in shape and more constricted 
at its inner part, so that the more dilated portion now clearly resembles the future 
retina, the constricted and tubular portion the optic nerve ; the more gradual con- 
traction of this protrusion and separation of it from the cerebral mass is also now 
more clearly seen. The cavity, which on the second day existed in the cerebral cell 
and communicated with the protrusion, is now closed in, with the exception of a 
distinct fissure, which, commencing by curved borders at the margin of the lens, was 
seen to be continued inwards in the direction of the cerebral mass through the 
tubular portion of the protrusion, the optic nerve. 
The fissure seems to be evidently connected (as Bischoff also supposes) with the 
separation which is effected between the ocular vesicle and the pedicle by which it is 
connected with the cerebral cell, the fissure at first being wide and extending as far 
as the anterior part of the eye, at its inferior and inner side ; but as the tubular por- 
tion of the protrusion becomes more solidified and converted into the true optic nerve, 
the fissure becomes much narrowed. No doubt this is analogous to the fissure which 
exists permanently in the retina and optic nerve in Fishes, as is seen in the Cod. In 
the Turtle the fissure exists in the nerve though not in the retina, whilst in Birds it 
appears to be the same slit which is here described, through which the pecten gains 
admission into the interior of the eye. 
On the first half of the* fourth day (fig. 9) the whole organ seems to be increased 
in size : the exterior lamina was distinctly continuous all round, and was of a dark 
granular texture ; the second layer the retina presented a different appearance to 
* Anatomie Encyclopedie, tom. viii. p. 324, 
