FORMATION OF THE LEXS. 



765 



observations of Lieberkiihn that in mammals the fold -n'hich produces 

 the ocular cup or secondary vesicle runs back into the stalk so as to 

 fold in the optic nerve for a considerable space, and by the simul- 

 taneous intrusion of mesoblastic tissue, thus to lead to the introduction 

 of the central blood-vessels of the retina within the nerve. But in 

 birds,, according to the same observer, no such infolding of the stalk 

 occurs, so that in them the vessels are excluded from thenerve. The 

 malformation termed coloboma iridis is to be attributed to a persistence 

 of the choroidal cleft or fold, and the pecten of birds, close to the optic 

 nerve, with the vascular fold farther forwards, and the falciform fold of 

 the eyes of fishes are to be regarded as fibro-vascular structures formed 

 by original projection through the same fold. 



The further development of the parts of the eye may be briefly 

 stated as follows : — 



The expansion of the ocular cup continuing to proceed, the chamber 

 for the vitreous humour enlarges, and that structure gradually comes 

 to occupy its space between the retina and the lens. 



The marked distinction between the nervous and the pigmental 

 portions of the primitive ocular vesicle goes on increasing by the con- 

 tinued deposit of pigment in the latter, and its proportional thinning, 

 and by the great addition to the thickness and the textural differentia- 

 tion of the substance of the former. Thus the cells in the retinal or 

 nervous portion, by their rapid multiplication, soon l^ecome several 

 layers thick ; certain parts of these cells assume the spindle shape, and 

 exhibit elongation into fibres, while others retain the nuclear form, and 

 thus there is foreshadowed the division into the fibrous, ganglionic, and 

 nuclear layers of the retina. On the exterior a limiting membrane 



Fig. 5C>6. 



>>>. 



\ 



\ 



Fig. 566. — Section of the Ei'e of 



AN EjIERYO-CaLP of 30 MM. LONG, 



MAGNIFIED (froiu Julius Arnold). 



Tlie cavity of the lens is much 

 reduced in size from the increased 

 development of fibres from behind. 

 The intersection space begins to be 

 formed posteriorly, and the zone of 

 nuclei is thrown forward. The cornea 

 is now formed, covered externally by 

 the cuticular epithelium, and with the 

 separation of the aqueous chamber in- 

 ternally. Close to the lens is the 

 caiDsulo-pupillary membrane, which is 

 continuous with the vascular structure 

 occupying the secondary ocular capsule 

 or cavity for the vitreous humour, and 

 with the choroid membrane round the 

 mai-gin of the ocular capsule, where 



iris and ciliary processes will afterwards . ■-^" 



be formed. The layer of pigment is ■ 



now more developed, and the tissue of 

 the sclerotic is begun to be formed. ' • 



The eyelids are beginning to project as ' 



folds of the skin. 



makes its appearance, and in connexion with it the rudiments of the 

 cells composing the layer of rods and cones. The space between the 



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