DEVELOPMENT OF THE EYE. 



DEVELOPMENT OF THE EYE. 



The first development of the eye occurs as a hollow protrusion of the anterior 

 cerebral vesicle primary optic vesicle in the manner already mentioned (see 

 fig. G8, e, and fig. 03). The vesicle thus formed abuts externally against the 

 external epiblast of the side of the head (fig. 06) ; and this external epiblast opposite 

 the most prominent point of the primary op'ic vesicle, becomes thickened and in- 



-, 



Fig. 93, A. BRAIN OF CHICK OP 2ND DAT, 



VIEWED FROM BELOW, TO SHOW THE FORMA- 

 TION OF THE OPTIC VESICLES BY OUTGROWTH 

 OF THE SIDE OF THE FORE-BRAIN, AND AT 

 THE SAME TIME BY THE FOLDING OVER OF 

 THE ENLARGED PART, THE PRODUCTION OF 

 A GROOVING OR CUPPING OF THE VESICLES. 



(His.) 



f.lr., m.l)\, 7<.6r., fore-, mid-, and hind- 

 brain ; opt., optic vesicle ; i, infundibulum. 



fig. i3, B. BRAIN OF HCMAN EMBRYO OF 



THREE WEEKS, SHOWING THE PRIMARY 

 OPTIC VESICLES AS OUT GROWTHS FROM THE 

 FOREBRAIN. (His.) 



.Fig. 94. SIDE VIEW OF ANTERIOR 



PART OF BRAIN OF MORE ADVANCED 

 HUMAN EMBRYO, SHOWING THE 

 PRIMARY OPTIC VESICLE FOLDED 

 AND CUPPED. (His.) 



c Ji, cerebral hemisphere (part of) ; 

 ilf. , olfactory lobe ; opt., optic cup. 



C.~hs 



Wet,. 



Fig. 95. SIDE VIEW OF THE SAME PART OF 



THE BRAIN IN A STILL MORE ADVANCED 

 EMBRYO, THE EYE HAVING BEEN CUT AWAY. 



(His.) 



opt., cut end of optic stalk, showing the 

 manner in which it is folded ; i, iufundibu- 

 lum ; olf.p. , posterior part of olfactory lobe ; 

 olf.a., anterior part of the same ; c.h., cere- 

 bral hemisphere ; t.c., tubes cinereum. 



vau-imited, so as to form at first a hollow cup-shaped depression with thickened walls 

 (figs. 07. 08), and subsequently by the closing in of the epiblast at the mouth of the 

 cup. a hollow island of epithelial cells (fig. 90). This island, which is the rudimentary 

 lens, lies between, but is entirely distinct from the external epiblast on the one hand, 

 and the neural epiblast of the primary optic vesicle on the other hand. Its forma- 

 tion is accompanied by a cupping in of the primary optic vesicle (figs. 04, 07, 08), 

 which is invaginated before it, and this invagination is increased by an ingrowth 

 of mesoblast, which occurs between the lens and the cupped optic vesicle, and 

 which subsequently forms the vitreous humour. Invaginated in this way the 

 cavity of the original optic vesicle becomes almost entirely obliterated, and appears 

 merely as a cleft between the two layers which form the wall of the so-called " optic 

 cup." The inner of these two layers is from the first thicker than the outer, and in 

 it are developed all the parts of the future retina from the meinbrana limitans interna 



G 2 



