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HAND-BOOK OF PHYSIOLOGY. 



By the ingrowth of the lens the anterior wall of the primary optic vesicle 

 is forced back nearly into contact with the posterior, and thus the primary 

 optic vesicle is almost obliterated. The cells in the anterior wall are 



ix v 



FIG. 449. Longitudinal and vertical diagrammatic section of a Vertebrate brain. Letters as be- 

 fore. Lamina terminalis is represented by the strong black line joining Pn and Py. (Huxley.) 



much longer than those of the posterior wall; from the former the retina 

 proper is developed, from the latter the retinal pigment. 



The cup-shaped hollow in which the lens is now lodged is termed the 

 secondary optic vesicle: its walls grow up all round, leaving, however, a 

 slit at the lower part. 



FIG. 450. Longitudinal section of the primary optic vesicle in the chick magnified (from Remak). 

 A, from an embryo of sixty -five hours; B, a few hours later; C, of the fourth day; c, the corneous 

 layer or epidermis, presenting in A, the open depression for the lens, which is closed in B and C ; Z, 

 the lens follicle and lens; pr, the primary optic vesicle; in A and B, the pedicle is shown; in C, the 

 section being to the side of the pedicle, the latter is not shown ; v, the secondary ocular vesicle and 

 vitreous humor. 



Choroidal Fissure. Through this slit (Fig. 452), often termed the 

 cJioroidal fissure, a process of mesoblast containing numerous blood- vessels 

 projects, and occupies the cavity of the secondary optic vesicle behind 

 the lens, filling it with vitreous humor and furnishing the lens capsule 

 and the capsulo-pupillary membrane. This process in Mammals projects, 

 not only into the secondary optic vesicle, but also into the pedicle of the 

 primary optic vesicle invaginating it for some distance from beneath, 

 and thus carrying up the arteria centralis retince into its permanent posi- 

 tion in the centre of the optic nerve. 



This invagination of the optic nerve does not occur in bird$, and con- 

 sequently no arteria centralis retinae exists in them. But they possess 

 an important permanent relic of the original protrusion of the mesoblast 

 through the choroidal fissure, forming the pecten, while a remnant of the 



