140 EMBRYOLOGY OF THE LOWEK VERTEBRATES CH. 



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 

 S&S through the choroid fissure. This 

 mass of mesenchyme develops a 



Fn;. 78. Semidiagrammatic figure of the network of blood-VCSSels Continuous 



bisected eye of a Vertebrate embryo w ith those of the surrounding tissue. 



(liana o mm.) to show the course ot T ., - \r j v 



the optic nerve-fibres (after Asshetou, In the more primitive \ ertebrates 



1892). this mesenchymatous mass reaches 



c.f, Wall of choroid fissure ; ,,, ganglion-cell ; 110 great development but ill the 



indifferent, supporting, <-.-n ; /, lens; ./, Teleostei and the Sauropsida, the 



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 ( Teleost ei ), 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 ai all. It is a slender drawn-out portion 

 of the brain analogous with the olfactory tract of a teleostean tish. con- 

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

 i/ed portion which h;i> become converted into the optic cup. Its 

 function bem;j a conducting one the main mass of this stalk-like 

 portion df the brain is composed of white substance or ner\ 'e-libres. 



Th< lead of parsing oiit\\ard< over the rim of the cup 



