. Quain's elements of anatomy . ervous partof the eye, viz., the retina and optic nerve, together with the pig-mental layer which comes to lie external to the retina. This foldingtakes place simultaneously with the development of the crystalline lens,which is the product of an involution of the cuticle or epiblast occurringon the outside of each primary optic vesicle. During the involution andenlargement of the lens, the wall of the primary optic vesicle comes tobe depressed and doubled in upon itself, so as to form a cup-like hollowtowards the exterior, the secondary optic vesicle, into which
. Quain's elements of anatomy . ervous partof the eye, viz., the retina and optic nerve, together with the pig-mental layer which comes to lie external to the retina. This foldingtakes place simultaneously with the development of the crystalline lens,which is the product of an involution of the cuticle or epiblast occurringon the outside of each primary optic vesicle. During the involution andenlargement of the lens, the wall of the primary optic vesicle comes tobe depressed and doubled in upon itself, so as to form a cup-like hollowtowards the exterior, the secondary optic vesicle, into which the lens isreceived, but without filling entirely its cavity. The outer plate orinvoluted portion of this cup or secondary optic vesicle becomes by 842 THE NERVOUS SYSTEM. its further development and histological differentiation the nervous partof the retina, while the remaining inner or proximal plate is convertedinto its pigmental layer, and the stalk becomes the optic nerve in con-nection with the brain. Fig. 747. Fig. Fig. 747.—Section through the 3?kont part of ths head of a lepidostbus embryo. (From Balfour.) al, alimentary tract -jth, thalamencephalon ; I, lens of tlie eye ; , the optic vesicle. Fig. 748.—Frontal section through the head of an embryo chick op 3 days AND 6 hours. (From Kolliker.) ^^0, stalk of the ocular vesicle in connection with the thalamencephalon; p, proximaland d, distal wall of secondary ocular vesicle; I, lens ; cj, vitreous body. The transition at the hne of inflection from the thick nervous part tothe thin pigmental part is quite sudden, and as soon as pigment cellsbegin to be developed a very marked distinction is percejDtible betweenthe latter and the nervous structure of the retina. These cells were Fig. 749.—Section op the com-mencing eye op an embryo inTHREE STAGES. (From E-smak.) A, commencement of the forma-tion of the lens I, by depression of apart of C, the corneous layer ; jj?-,the primitive ocular vesicle nowdoubled b
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