PEIMAEY DEVELOPMENT OF THE EYE. 



■63 



cellular constituents than the inner or that towards the brain, and 

 from this, as well as the increase of the inward folding, the original 

 cavity of the primary optic yesicle becomes in a great measure 

 obliterated or narrowed, and the outer and inner folds are closely 

 approximated, while the stalk or pedicle becomes i^roportionally much 

 diminished. The continued increase of cellular development in the 

 outer fold of this cup leads to the formation of the various elements 

 composing the retina ; while in the thinner inner fold only pigment cells 

 are formed. The transition at the bend from the thick nervous part tc 



Fi- 5G3 



g^.5^18 ^ 



Fig. 563. — Section thkottgh the Eye op an Em- 

 bryo-Calf OF TWELVE 3IM., OR HALF AN INCH, 

 LONG, MAGNIFIED (froui Julius Amold). 



The lens follicle Ls now closed in and detached 

 fi'ora the corneous laj'er, and its cavity contains loose 

 cells which are the remains of the superficial corneous 

 cells. The oj^tic vesicle or cajisule is now completely 

 doubled back, so as to present towards the lens side 

 the secondary ocular capsule or cup ; its outer wall 

 now much thickened by the commencement of the 

 development of retina. Mesoblastic tissue is seen 

 to have passed in from the periphery between the 

 optic capsule and the lens, as well as in front of the 

 lens. The commencement of a vascular circle is 

 shown round the exterior of the ocular capsule, and 

 extending also between the lens follicle and the 

 cuticle. 



the thin pigmental part is quite sudden, and as soon as pigment cells 

 begin to be developed, a very marked distinction is perceptible between 

 it and the nervous structure of the retina. These cells were formerly 

 regarded as a part of the choroid membrane, but they are now looked 

 upon as belonging rather to the retina, — a view which is supported by 

 the mode of development now described. 



Fig. 56i. — Diagrammatic Sketch op a 

 Vertical Longitudinal Section 



THROUGH THE EyEBALL OP A HuMAK 



FcETus OF FOUR WEEKS (after KoUi- 

 ker). ^^ 



The section is a little to the side so as to 

 avoid passing through the ocular cleft, c, 

 the cuticle, where it becomes later the 

 cornea ; I, the lens ; o ij, optic nerve 

 formed by the pedicle of the primary oj)tic 

 vesicle ; v 2'>, primary medullary cavity of 

 the optic vesicle ; p, the pigment-layer of 

 the outer wall ; r, the inner wall form- . 

 ing the retina ; v s, secondary optic vesicle 

 containing the rudiment of the vitreous 

 humour. 



The fold which produces the optic cup proceeds from above down- 

 wards, and Avhen the lens is formed it seems as if it enclosed the lens 

 but left for a time an aperture or depression below. This is the 

 choroidal fold or fissure, which may easily be distinguished in the 

 emliryo-head after pigment has been deposited, fi'om the circumstance 

 that the pigment is absent from the cleft, which thus appears for a 



