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, PI. IV. fig. 16), separates by a slight fissure into two layers, constituting the primary 

 vesicle (PI. 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 

 (PI. IV. figs. 14, 21), whose chamber — the lumen of the secondary vesicle — is closed 

 in front by the growing lens (I). 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 (PI. XIV. fig. 1; PI. XVII. fig. 10). On each 

 side of the lens they are densely aggregated (PI. V. fig. 6 ; PL XVI. fig. 8). The 

 outermost layer of the cellular vesicle, i.e., the stratum of cells internal to the layer of 

 pigment, assumes a marked columnar character (PL XL figs. 6-8) — bold striae passing 

 across it, and dividing it into wedge-shaped radiate masses as indicated in PL XXIII. fig. 3«. 

 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 

 Hoffman, 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 laminae. The columnar character of the bacillar stratum is still more 



* 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. 



t This layer Dr Gunn has identified as the "external molecular layer" — a "thin dark finely granular line;" 

 and should this he so, then the " limitcms externa" must he developed at a much later stage, as Dr Gunn states. — Vide 

 Ann, Nut. Hist., Sept. 1888, p. 268. 



