28o COMPARATIVE ANATOMY 



the choroid are formed from paired outgrowths of the fore- 

 brain known as the optic-vesicles. These vesicles appear 

 early in development, soon after the closure of the neural 

 folds, as tubular evaginations of the fore-brain. The evagina- 

 tions grow out towards the sides of the head, their distal 

 ends become swollen and their proximal ends constricted, 

 so that each has the form of a hollow bulb attached to the 

 fore-brain by an obliquely directed hollow stalk. The outer 

 half of the bulb — i.e. the half nearest to the surface epiblast — 

 then becomes thickened, and is first flattened and then folded 

 back within the inner thin-walled hemisphere, so that the 

 original cavity of the vesicle is nearly obliterated (fig. 64, A). 

 In this way a two-layered cup, the optic cup, is produced. 

 Meanwhile the nervous layer of the epiblast opposite the optic 

 cup has become thickened, so as to form a solid mass of 

 cells projecting into the mouth of the cup. The thickening 

 separates from the epiblast, acquires a central cavity, and 

 lies in the mouth of the optic cup. At first a hollow vesicle, 

 it becomes converted into the transparent lens by subsequent 

 thickening and histological differentiation of the cells forming 

 its posterior wall (fig. 70, £). 



The optic cup is surrounded by a mesoblastic investment, 

 the inner layers of which become the choroid and the outer 

 layers form the sclerotic. vVt the mouth of the cup the 

 mesoblastic investment pushes its way as a double layer 

 between the lens and the surface epiblast. The inner of 

 these two layers forms the iris, the outer gives rise to the 

 cornea, while the epiblast remains as the anterior corneal 

 epithelium. The thinner outer wall of the optic cup becomes 

 the pigmented epithelium of the choroid, and the thicker 

 inner wall lining the cavity of the cup becomes the retina. 

 The vitreous humour filling the cavity of the cup between 

 the retina and the lens is formed by an ingrowth of mesoblast 

 through a gap produced on the under side of the optic cup 

 by the peculiar way in which the primary optic vesicle is 

 infolded to form a two-layered cup. The stalk of the optic 

 cup is at first hollow, but its lumen is afterwards obhterated by 

 the outgrowth of the fibres of the optic nerve from the brain to 

 the retina. The full development of the eyes is not attained till 

 the close of the metamorphosis, these organs remaining in an 

 imperfect condition throughout the larval stage of existence. 



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