764 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



Teleostean embryo, as already noted, is the enormous development of the anterior 

 cephalic region, which is chiefly due to the protrusion of two rounded lateral masses from 

 the sides of the narrow fore-brain (PI. V. fig. 1, op), and not, as Lereboullet stated, 

 from the walls of the mid-brain (No. 93, p. 522). The pair of massive bulbs thus 

 formed are rapidly defined as the ellipsoidal optic vesicles, the first of the sensory 

 organs to appear. In section (PI. IV. fig. 1 6) the cells of the neurochord, at a point 

 midway between the dorsal and the ventral surface, actively push their way outward, 

 and pass for the most part upward, so that a pair of stalked vesicles are formed, lying 

 against the sides of the fore-brain — not quite upright, but placed at an angle which 

 brings the lower and smaller lobe in proximity, while the upper and much larger lobe is 

 pushed away from the brain (PI. IV. fig. 3). Sections clearly demonstrate the abundant 

 protrusion of cells to form the optic bulbs, which Kingsley and Conn regard as formed 

 in the main by a constriction or fissure commencing above and behind the lateral 

 enlargement, and progressing forward and downward (No. 78, p. 207), but the constriction 

 which they carefully describe is preceded by a very apparent bulbous outgrowth. These 

 protruded cells are indistinguishable in size or contour from the neurochordal cells which 

 gave them origin, but the outer limiting layer of cells assumes a columnar disposition, as 

 also does a double plate of cells along the median dorso-ventral plane. This latter feature 

 has been referred to (No. 122, p. 452) as a radial disposition of the central cells and 

 "as though about to dehisce along a central vertical plane in order to form a median 

 chamber, longitudinally placed ; " but a chamber converting the solid optic proliferations 

 into capacious hollow vesicles, such as the early condition of these structures is generally 

 described, is never completed — a very narrow fissure being all that is usually formed, and 

 even this may at times fail to be developed before the invagination of epiblast presses 

 the outer layer against the inner layer of the primitive optic vesicle. Ryder describes 

 and figures the narrow fissure referred to (No. 141, p. 499, pi. v. figs. 26, 27); and 

 in section (PI. IV. fig. 17) it is plainly seen as a slit in the midst of the optic vesicle. 

 This separation of the median cells is interesting, for, though the Teleostean eye is not 

 pushed out as a chambered sac from a hollow brain-vesicle, as is the primitive mode of 

 origin, it secondarily acquires a trace of this vesicular condition. In the living embryo 

 it rarely presents more than the character of a delicate median line or slit in the optic 

 bulb (PI. V. fig. l). PI. IV. fig. 3, shows the first indication of this slit-like lumen, 

 which can be traced along the short thick stalk into the fore-brain, where it is lost. In 

 horizontal section we see that while the cells — pushed out to form the optic vesicle — in the 

 main pass upward, they also extend posteriorly, carrying the vesicle some distance behind 

 its pedicel or point of origin: the optic vesicles on their appearance are thus defined 

 most distinctly behind. Each vesicle, in fact, forms a depressed pyriform body, 

 which by its smaller end remains attached to the brain, while the swollen upper 

 portion extends dorsally, backward and distally (PI. IV. fig. 16). Schenk, who first 

 gave a full account in his well-known researches on the eye of fishes (No. 143), seems 

 not to have recognised the fact that the eye and the entire central nervous system in 



