[45] EMBRYOGRAPHY OF OSSEOUS FISHES. 499 



neurula, which are in direct conuection with it from the first, are the 

 optic vesicles or rudiments of the eyes. They are the first of the sensory 

 structures of the young fish to be developed. They, in fact, are already 

 apparent at a very early stage of development, and are formed as lat- 

 eral outgrowths of the extreme anterior end of the neurula, after the 

 eighth daj', and by the ninth day, as shown in Figs. 20 and 21, op, they 

 are conspicuous as thickened lateral lobes of the anterior portion of the 

 cephalic end of the rudimentary neural system. Their development in 

 Teleostean embryos is to a certain extent characteristic, in consequence 

 of their great relative size in proportion to other parts of the nervous 

 system at this time. At first their connection with the neurula is quite 

 lateral and anterior; as development proceeds, however, the down 

 growth of the carina or keel of the neurula carries their stalks or points 

 of attachment downward. The place where they arise from the keel 

 marks the position of the origin of the optic nerves, in the vicinity where 

 the cerebrum is continued into the more posterior portions of the brain 

 or thalam-encephalon. During the early stages of their development, 

 they, like the neurula itself, are composed of a solid depressed ovoidal 

 mass of cells. As development proceeds, this mass acquires a lumen 

 or cavity, which is at first a mere cleft like the primitive cerebral lumen 

 or cavity. The cavity of the optic vesicles in section is at first some- 

 what oblique to the plane of the blastoderm, but this feature is lost with 

 advancing development, and the lower wall of the optic vesicle is finally 

 pushed inward, upward, and more towards the axis of the embryo, while 

 the hind wall itself is also raised so that both together assume a more 

 nearly vertical position. This condition is shown Fig. 27, taken from 

 the head of an embryo eleven days old. The rudiment of the eye-ball 

 is now more nearly vertical and oval as seen from the side. It is now 

 a very depressed double-walled cup as viewed from above, and is con- 

 nected by a hollow stalk with the lower forward part of the brain. In 

 Fig. 27 its stalk lies just behind and below the rudiment of the nasal 

 or olfactory pit na, and its outer lamina op is afterwards transformed 

 into the retina, while the inner thinner lamina becomes covered by the 

 choroid or pigmented layer, and the thickened epithelial tract of epi- 

 blast I is carried inwards with the further development of the eye-ball, 

 and transformed by invagination and further metamorphosis into the 

 lens. The central part of the thickened rudiment I, of the lens, becomes 

 the transparent, posterior highly refractive fibrous part, while the sur- 

 rounding thinner margin of the layer I is reflected over the thicker 

 hinder part, as a thin layer of epithelial cells. After this the lens is 

 constricted oft' from the epithelium and the construction of the eye is 

 essentially comideted. The further development, however, of the eye 

 involves the consideration of still other events in the history of its layers. 

 The development of the lens is shown in Figs. 26, 27, 29, and 30. In Fig. 

 29 the outer epithelial layer is shown, inclosing, as it were, the columnar 

 internal refringent layer within. The optic cup is still open below on 



