ORGANS OF VISION OF THE VERTEBRATA. 



411 



The lens. In the majority of groups, viz. Elasmob ranch! i, Reptilia, 

 Aves, and Mammalia, the lens is 

 formed by au open invagination of 

 the epiblast, but in Amphibia, Te- 

 leostei and Lepidosteus, where the 

 nervous layer of the skin is earlv 

 established, this layer alone takes 

 part in the formation of the lens 

 (tig. 293 I). The lens is however 

 formed even in these types as a 

 hollow body by an invagination; 

 but its opening remains perma- 

 nently shut ofl from communica- 

 tion with the exterior by the epi- 

 dermic layer of the epiblast. Gb'tte 

 describes the lens as formed by a 

 solid thickening of the nervous 

 layer in Bombinator. This is pro- 

 bably a mistake. 



The cornea. The mode of 



formation of the cornea already de- 

 scribed appears to be characteristic 

 of most Vertebrata except the Am- 

 moccete. It has been found by 

 Kessler in Aves, Reptilia and 

 Amphibia, and probably also occurs 

 iu Pisces. In Mammals it is not 

 however so easy to establish. There 



are at first no mesoblast cells be- >'2 } - retinal pigment, 

 tween the lens and the epiblast (tig. 



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 cornea! 

 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 lamina? of the cornea. The cornea continues to increase 

 in thickness by the addition of lamina; 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. 



During the third day of incubation there passes in through the choroid 

 slit a vascular loop, which no doubt supplies the transuded material for 

 the growth of the vitreous humour. Up to the fifth day this vascular 



FIG. 292. EYE OF AN AMMOCIETES LYING 

 BENEATH THE SKIN. 



ep. epidermis ; dc. dermal connective 

 tissue continuous with the sub-dermal con- 

 nective tissue (.s-.rf.c), which is also shaded. 

 There is no definite boundary to this tissue 

 where it surrounds the eye. 



in. muscles; dm. membrane of Descemet; 

 /. lens; r.h. vitreous humour; r. retina; 



