190 MR. H. GRAY Oj'T THE DEVELOPMENT OF THE RETINA AND OPTIC NERVE, 
dilated end, bat no protrusion can be observed in the situation where the eye is to be 
subsequently formed. At the thirty-third hour (fig. 2) there is observed on each side 
of the cephalic extremity a protrusion of its walls, which at the thirty-sixth hour (fig. 3) 
has very much increased, having become more elongated and protruded outwards, 
presenting a somewhat dilated end, and being slightly constricted at its connection 
with the cerebral cell from which it arises ; the whole embryo is enlarged, and now 
there is a distinct division of the brain into its several primitive cells ; this protrusion, 
which is well represented in fig. 3, is the first distinct indication of the mode of deve- 
lopment of this membrane. 
In an embryo chick, examined at the forty-sixth hour of incubation from the cere- 
bral aspect (figs. 4 and 5), the future brain consisted of several cells ; the anterior, 
which was the largest, corresponded to the cerebral lobes ; the middle, smaller in size, 
to the future optic lobes ; and the posterior, the smallest, but most elongated, to the 
medulla oblongata. From the most anterior one there arose a protrusion on each side, 
having a somewhat dilated extremity : this protrusion I will call the optic vesicle. The 
cavity of the cerebral cell from which it arose was very distinct and its wall clear and 
pellucid, and appeared to communicate with the optic vesicle, which was also hollow. 
As the cavity of the cerebral cell passed outwards on both sides into the optic 
vesicle it became less distinct, as there the wall of the vesicle, which is darkly gra- 
nular, makes it in this situation less apparent, whilst the cavity in the cerebral cell 
itself was distinctly seen from the thinness of its wall on its ventral aspect. If the 
embryo is examined from its dorsal surface (its natural position in the egg), the cere- 
bral cell presents an external convex surface, whilst projecting from each side are 
seen the optic vesicles. The cavity of the cell and its communication with the optic 
vesicle, cannot on this surface be distinctly demonstrated on account of its wall being 
thicker than on the ventral side. The optic vesicle is bounded externally by a well- 
defined line which lines the outer surface of the protrusion, and seems to be con- 
nected with the envelopes of the cerebral mass ; this is again bounded by the tegu- 
mentary layer of the embryo. When examined with a high magnifying power, the 
vesicle presented a pale granular texture without any indication of cellular structure. 
This description of the origin of the retina, although confirmed by similar observa- 
tions, described by Baer in the Encyclopedie Anatomique, tom. viii., is not in accord- 
ance with that of some other celebrated physiologists. According to Wagner, the pro- 
trusion which forms the future retina arises from the middle and anterior cerebral cells, 
whilst FIuschke describes it as “a simple rudiment, consisting of a fosette ; that the 
dorsal laminae form in front of their anterior dilatation, the first cerebral cell ulti- 
mately dividing this into two lateral halves, from which result the two rudimentary 
eyes.” He says also that this fosette is formed before the end of the first day. From 
many observations I have made I could never see what these observers have de- 
scribed, and, coinciding as my observations do so exactly with those of Baer, the 
conclusions which I have stated regarding the origin of this membrane would seem 
