Optic vesicles. The development of the primitive optic vesicles, so far as is known, is very constant throughout the Vertebrata. In Teleostei and Lepidosteus alone is there an important deviation from the ordinary type, dependent however upon the mode of formation of the medullary keel, the optic vesicles arising while the medullary keel is still solid, and being at first also solid. They subsequently acquire a lumen and undergo the ordinary changes.
Fig. 292. Eye of an Ammocœtes lying beneath the skin.
ep. epidermis; d.c. dermal connective tissue continuous with the subdermal connective tissue (s.d.c), which is also shaded. There is no definite boundary to this tissue where it surrounds the eye.
m. muscles; dm. membrane of Descemet; l. lens; v.h. vitreous humour; r. retina; rp. retinal pigment.
The lens. In the majority of groups, viz. Elasmobranchii, Reptilia, Aves, and Mammalia, the lens is formed by an open invagination of the epiblast, but in Amphibia, Teleostei and Lepidosteus, where the nervous layer of the skin is early established, this layer alone takes part in the formation of the lens ([fig. 293], l). The lens is however formed even in these types as a hollow body by an invagination; but its opening remains permanently shut off from communication with the exterior by the epidermic layer of the epiblast. Götte describes the lens as formed by a solid thickening of the nervous layer in Bombinator. This is probably a mistake.
The cornea. The mode of formation of the cornea already described appears to be characteristic of most Vertebrata except the Ammocœte. It has been found by Kessler in Aves, Reptilia and Amphibia, and probably also occurs in Pisces. In Mammals it is not however so easy to establish. There are at first no mesoblast cells between the lens and the epiblast ([fig. 295]) but in many Mammals (vide Kessler, No. [372], pp. 91-94) a layer of rounded mesoblast cells, which forms Descemet’s membrane, grows in between the two, at a time when it is not easy to recognise a corneal lamina, as distinct from a simple coagulum.
After the formation of this layer the mesoblast cells grow into the corneal lamina from the sides, and becoming flattened arrange themselves in rows between the laminæ of the cornea. The cornea continues to increase in thickness by the addition of laminæ on the side adjoining the epiblast.
We have already seen that in the Lamprey the cornea is nothing else but the slightly modified and more transparent epidermis and dermis.
The optic nerve and the choroid fissure. It will be convenient to consider together the above structures, and with them the vascular and other processes which pass into the cavity of the optic cup through the choroid fissure. These parts present on the whole a greater amount of variation than any other parts of the eye.
I commence with the Fowl which is both a very convenient general type for comparison, and also that in which these structures have been most fully worked out.