766 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
section, at a lower plane (PI. IV. fig. 19), shows the fissure disappearing. These intruding 
mesoblastic cells (mes) appear to become packed between the lens and the retinal surface 
of the optic cup, and doubtless break down to constitute the vitreous humour of the adult 
eye, forming also, as some observers think, the “capsula hyaloidea,” in which a rich 
vascular network afterwards develops. The differentiation of the cellular optic vesicle 
into its various layers has already taken place before the embryo has emerged from the 
ovum. The formation of these layers can, however, only be very briefly touched upon.* 
We have seen that the eye, soon after its appearance as a solid bulbous protrusion 
(op, Pl. IV. fig. 16), separates by a slight fissure into two layers, constituting the primary 
vesicle (Pl. IV. fig. 17). With the obliteration of the lumen, the two layers become 
closely apposed, and the eye consists of a thick-walled cup of undifferentiated cells 
(Pl. IV. figs. 14, 21), whose chamber—the lumen of the secondary vesicle—is closed 
in front by the growing lens (/). An investing layer of mesoblast (mes) forms the 
sclerotico-choroidal sheath, absent, however, from the front of the eye. As the time for 
extrusion approaches, scattered pigment-spots occur outside the optic vesicle and in the 
external investment. These spots are unbranched amorphous particles, sparsely 
distributed as an irregular pigment-layer over the whole surface of the optic cup, save 
in front of the aperture of the pupil (Pl. XIV. fig. 1; Pl. XVII. fig. 10). On each 
side of the lens they are densely aggregated (Pl. V. fig. 6; Pl. XVI. fig. 8). The 
outermost layer of the cellular vesicle, 7.e., the stratum of cells internal to the layer of 
pigment, assumes a marked columnar character (Pl. XI. figs. 6—8)—bold striz passing 
across it, and dividing it into wedge-shaped radiate masses as indicated in P]. XXIIL fig. 3a. 
At the same time the cells within, constituting the main bulk of the vesicle, separate, 
though somewhat obscurely, into two layers of great and almost equal thickness—the inner 
layer being slightly thicker (in section) than the outer. The line of separation is delicate 
and indistinct at first, but subsequently develops into a fine molecular band—the prominent 
internal molecular layer. The inner surface of the columnar stratum shows a delicate 
membrane, possibly the posterior or membrana limitans externa (“limitans interna” of 
Horrman, No. 69, p. 50).t From the outer stratum consisting of columnar elements 
the rods and cones are developed, while the two thick inner layers with their intervening 
lamina give rise to the other layers of the retina. Such is the condition of the six 
layers of the retina shortly before the time of hatching in pelagic forms, e.g., the cod and the 
haddock. In other forms, chiefly demersal, which reach a somewhat advanced embryonic 
stage while within the ovum, the eye attains a much further degree of development. 
A haddock, on the second or third day after extrusion, shows additional changes, the 
second layer being better marked, as is also the inner molecular layer, though both 
are still very thin amine. The columnar character of the bacillar stratum is still more 
ce 
* The minute description of the development of the Teleostean eye is in the able hands of Dr Marcus Gunn, one 
of the surgeons at Moorfields. 
+ This layer Dr Gunw has identified as the “external molecular layer”—a “thin dark finely granular line ;” 
and should this be so, then the “imitans externa” must be developed at a much later stage, as Dr Gunn states.— Vide 
Ann. Nat. Hist., Sept. 1888, p. 268, 
