500 THE CORNEA. 



layer of the epiblast. Gotte 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 Ammocoete. 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 laminae of the cornea. The cornea continues to 

 increase in thickness by the addition of laminae 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 con- 

 venient 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 loop is 

 the only structure passing through the choroid slit. On this day however a 

 new structure appears, which remains permanently through life, and is 

 known as the pec ten. It consists of a lamellar process of the mesoblast 

 cells round the eye, passing through the choroid slit near the optic nerve, 

 and enveloping part of the afferent branch of the vascular loop above 

 mentioned. The proximal part of the free edge of the pecten is somewhat 

 swollen, and sections through this part have a club-shaped form. On the 

 sixth day the choroid slit becomes rapidly closed, so that at the end of the 

 sixth day it is reduced to a mere seam. There are however two parts of 

 this seam where the edges of the optic cup have not coalesced. The 

 proximal of these adjoins the optic nerve, and permits the passage of the 

 pecten and at a later period of the optic nerve ; and the second or distal one 

 is placed near the ciliary edge of the slit, and is traversed by the efferent 

 branch of the above-mentioned vascular loop. This vessel soon atrophies, 

 and with it the distal opening in the choroid slit completely vanishes. In 

 some varieties of domestic Fowl (Lieberkiihn) the opening however persists. 

 The seam which marks the original site of the choroid slit is at first 



