DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 765 
these forms is primarily solid and without a lumen. Kuprrer appears to have been the 
first to describe the true condition (No. 88). 
It is important to notice that the nerves—that is, the stalks of the optic vesicles—arise 
at a different level from the olfactory and other nerves, a fact inconsistent with the 
derivation of the nerves from a ridge or “ sinnesplate,” such as GOrre and others 
distinguish, A dorsal stratum of neurochordal cells may perhaps be regarded as a 
neural crest from which the posterior nerves spring, but such a crest does not pass 
further forward than the hind-brain ; the first and second pair of cranial nerves, as will be 
seen, arise, the one primarily as dorsal and the other as lateral median evaginations of the 
prosencephalon. In pushing backward the optic vesicles shut off a thin stratum probably 
of mesoblast which later forms an enveloping cup, and gives origin to some important 
structures in the developing eye. This mesoblast (mes, Pl. IV. fig. 17) is probably a 
forward growth from the thin plate of the same layer in the otocystic region (Pl. IV. 
figs. 16, 17). Meanwhile, the short, thick connecting stalk becomes constricted, and 
the vesicle itself alters both in form and position. Viewed from the side, the latter 
is now almost perfectly elliptical (op., Pl. XXII. fig. 12), and is nearly perpendicular in 
position, z.e., parallel to the vertical axial plane of the embryo (Pl. V. figs. 3, 10). 
The columnar cells along the central vertical plane of each vesicle (Pl. IV. fig. 3) 
separate sufficiently to mark a slight but distinct fissure (Pl. IV. fig. 17; Pl. V. 
fig. 1). This fissure persists when the optic vesicles have altered their position, so that they 
lean by their upper portion against the neurochord, and this median chamber, instead of 
passing upward, outward, and posteriorly, as when first indicated (PI. V. fig. 1), now 
passes downward and outward (op, Pl. IV. fig. 14). As already indicated, the pyriform 
outline is almost wholly lost, the optic vesicles lying obliquely against the fore and mid- 
brain—as elliptical bodies laterally flattened, and traversed by a vertical lumen longi- 
tudinally separating each into an inner and an outer half, the latter layer being very 
much thicker than the inner half (PI. IV. fig. 17). This condition does not remain long. 
Before the end of the day these “ primitive optic vesicles ” become indented by the pressure 
of the epiblast lying external to them, the deeper layer of which becomes rapidly thickened 
so as to form in section (PI. IV. fig. 17) an almond-shaped mass on each side, pressing upon 
the central region of each optic vesicle (op), which gradually becomes cup-shaped, the hollow 
of the eup being occupied by the thickened mass of epiblast, which forms a dense spherical 
body, the lens (/, Pl. IV. fig. 14). The optic cup or secondary optic vesicle becomes thinner 
marginally, and this portion creeps round to the outer side of the lens (Pl. 1V. fig. 21), 
forming a circular lip around it, which is incomplete on the lower side. This gap, the 
choroidal fissure, is very distinctly seen at this stage (ch, Pl. VUI. figs. 6, 7, 8; Pl. 
IX. figs. 1, 3; Pl. XII. figs. 1, 2), and it persists for some time (Pl. XVI. fig. 1). 
The mesoblastic cells, which were included as a thin plate between the optic 
vesicle and the brain (mes, Pl. IV. fig. 17), have spread over the former as an outer 
layer (Pl. IV. fig. 21), and pushed their way through the choroidal fissure into the 
interior chamber of the eye, as is seen in section (Pl. IV. fig. 20). A similar horizontal 
