DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 765 



these forms is primarily solid and without a lumen. Kupffee 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 Gotte 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 (PI. IV 7 . 

 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, i.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; PI. 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 (PL V. fig. ]), now 

 passes downward and outward (op, PI. 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 (PL 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 (PL 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 cup being occupied by the thickened mass of epiblast, which forms a dense spherical 

 body, the lens (I, 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 IV. 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 VIII. figs. 6, 7, 8; PL 

 IX. figs. 1, 3; PL XII. figs. 1, 2), and it persists for some time (PI. XVI. fig. l). 



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 (PI. 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 



