140 EMBEYOLOGY OF THE LOWEK VERTEBEATES oh. 



along the deep surface of Descemet's membrane and there settle 

 down to form a single layer of flattened cells. On the deep side of 

 this corneal endothelium a split gradually develops in the jelly-like 

 matrix : this contains a watery secretion (aqueous humor) and 

 becomes the anterior chamber of the eye. The portion of matrix 

 lying superficial to Descemet's membrane becomes colonized by cells 

 from all round its margin. It forms the main portion of the cornea, 

 while a thin layer lying next the ectoderm remains uncolonized and 

 gives rise to Bowman's membrane. 



Vitreous Body. — The cavity of the optic cup is from the beginning 



filled with clear fluid which keeps 

 it distended and there is no ap- 

 parent reason to assume that this 

 arises otherwise than by the same 

 method as holds with the eyes of 

 many invertebrates i.e. as a secre- 

 tion of the surrounding retinal cells. 

 The fluid gradually acquires the 

 jelly-like consistency characteristic 

 of the fully-formed vitreous body. 



Amoebocytes wander at a com- 

 paratively early period into the 

 vitreous rudiment — in the Fowl 

 embryo about the third day — and at 

 a later period a continuous mass of 

 mesenchyme tissue projects into it 

 through the choroid fissure. This 

 mass of mesenchyme develops a 

 network of blood-vessels continuous 

 with those of the surrounding tissue. 

 In the more primitive Vertebrates 

 this mesenchymatous mass reaches 

 no great development hut in the 

 Teleostei and the Sauropsida, the 

 most highly specialized groups 

 amongst the non-mammalian Verte- 

 brates, it does so and persists throughout life, as the falciform 

 process with its muscle-fibres for the purpose of accommodation in 

 the one case (Teleostei), and the highly vascular, and probably mainly 

 nutritive, pecten in the other ( Sauropsida). 



Optic Nerve. — As already indicated the optic nerve is not strictly 

 speaking a peripheral nerve at all. It is a slender drawn-out portion 

 of the brain analogous with the olfactory tract of a teleostean fish, con- 

 necting the main portion of the brain with the small highly special- 

 ized portion which has become converted into the optic cup. Its 

 function being a conducting one the main mass of this stalk-like 

 portion of the brain is composed of white substance or nerve-fibres. 

 These fibres instead of passing outwards over the rim of the cup 



Fig. 78. — Semidiagrammatic figure of the 

 bisected eye of a. Vertebrate embryo 

 (Rana 8 mm.) to show the course of 

 the optic nerve-fibres (after Assheton, 

 1892). 



c.f t "Wall of choroid fissure ; g, ganglion-cell ; 

 i, indifferent, supporting, cell ; I, lens ; n.f, 

 nerve-fibres : p.l t pigment layer of retina ; 71, 

 percipient cell. 



