DEVELOPMENT OF THE EYE. 



85 



place where the lens is becoming involuted, but also below, or ventral, to that place, 

 so that a section exactly through the middle of the optic cup at right angles to the 

 axis of this part of the head, shows a gap in the boundary of the cup through which 

 the mesoblast is passing into the space between the lens and the invaginated optic 

 vesicle (fig. 98, v). This gap or cleft soon becomes closed, but the suture or line of 

 closure long remains apparent from the fact that when pigment begins to be de- 

 posited in the eye, this so-called choroidal fissure remains for some time unpigmented 

 (until the sixth week in man). 



The ventral invagination is in mammals continued for a considerable distance 

 into tho stalk of the optic vesicle (fig. 95), and the simultaneous inclosure of 

 mesoblastic tissue leads to the introduction of the central blood-vessels of the retina 

 within the optic nerve. In birds no such infolding of the stalk occurs. 



The lower invagination of the optic cup serves not only to permit of the passage of meso- 

 blast behind the lens for the formation of vitreous humour, but also to establish a direct 

 connection between the nerve-fibres which are formed along the course of the optic stalk 

 (future optic nerve) and the centre of the inner layer of the optic cup (future retina) (0. 

 Hertwig). 



The malformation termed coloboma iridis is attributed to a persistence of the choroidal 

 cleft, which extends behind the iris along with the retinal pigment or uvea, as far as the 

 margin of the pupil. 



Fig. 100. HORIZONTAL SECTION THROUGH THE EYE 



OF AN EMBRYO RABBIT OP TWELVE DAYS AND SIX 



HOURS. 7 1 . (Kolliker. ) 



o, optic stalk ; h', remains of the cavity of the 

 primary optic vesicle ; p, proximal lamella of the 

 optic cup (pigmentuin nigrum) ; r, distal lamella 

 (retina) ; I, lens invagination. widely open at ol ; 

 Z', papillar elevation in the bottom of the lens 

 vesicle ; m, mesoblast ; g, mesoblast of vitreous ; 

 v, a blood-vessel at the anterior border of the optic 

 cup ; e, cutaneous epiblast. 



Fig. 101. EYEBALL OF A HUMAN EMBRYO OF 



FOUR WEEKS CUT ACROSS, AND THE ANTERIOR 

 HALF REPRESENTED FROM BEHIND. (Kolliker. ) ^ 



pr, the remains of the cavity of the primary 

 optic vesicle ; p, outer layer forming the retinal 

 pigment ; r, the thickened inner part giving rise 

 to the columnar and other structures of the retina ; 

 v, commencing vitreous humour within the optic 

 cup ; v', the cleft through which a vascular loop, 

 a, projects from below ; /, the lens with a central 

 cavity. 



The hollow optic stalks are at first freely in communication with the thaiam- 

 encephalon, or third ventricle. Nerve-fibres grow along their walls, from neuroblasts 

 which develop in the retinal epiblast, and pass towards the nerve-centre (His), and 

 the cavities of the stalks become thereby gradually obliterated, the radially striated 

 epithelial-like arrangement of the wall being, however, long evident. A new con- 

 nection becomes subsequently established between the posterior part of the optic- 

 stalks (optic tracts) and the mesencephalon, whilst the middle parts become united 

 with one another to form the chiasma. 



