140 EYE 



successive layers and in three (or more) separate sections, so that their ends abut 

 against one another in front and behind along tri-radiate (or multi-radiate) lines, 

 such as may be seen in the macerated lens. These later deposited fibres are all 

 formed at the equator (at the transitional zone), where cell-multiplication chiefly 

 takes place, and they grow hence meridionally backwards over the ends of the 

 already developed antero-posteriorly disposed fibres of the central part of the lens. 



Development of the optic cup. The doubling- in of the optic vesicle is 

 a gradual process of involution, and from the first the invaginated outer wall is 

 thicker than the inner (fig. 182). The thinner outer layer of the completed cup 

 early shows a deposit of pigment, and becomes the hexagonal pigmented 

 epithelium of the retina, while the thicker inner layer is converted by a 

 complicated series of changes into the retina. 



The optic vesicle at first opens into the cavity of the fore-brain by a wide 

 aperture. As the vesicle enlarges the lips of this gradually close in, and the stalk 

 becomes elongated into a hollow cord. The upper wall of this tube is thinner 

 than the lower. When the optic cup is formed the thin upper wall of the stalk is 

 continued into the outer layer, while the thick lower wall is continued into the 

 thick retinal layer of the cup. This is due to the character of the invagination of 

 the optic vesicle. It is not a simple in-pushing of the outer wall by the growing 

 lens-vesicle, for the folding is not confined to the part of the wall against which 

 the lens lies, but also implicates the ventral wall and commencement of the stalk 

 (fig. 178). A cleft is thus left below the lens which is continued some distance 

 along the stalk as a fold of its thick lower wall (fig. 179). The cleft and groove 

 soon become closed in, but before this is effected vessels enter the hollow of the 

 cup (fig. 182), the fate of which will be discussed later. 



The line of closure of the lips of the cleft remains apparent for some time owing to the fact 

 that when pigment develops in the wall of the cup this so-called choroidal fissure remains 

 unpigmented for a time. The malformation known as coloboma iridis is attributed to a 

 persistence of this fissure or unpigmented tract. 



Development of the retina. The thickened inner layer of the optic cup 

 early shows a distinction into a thicker posterior portion, the pars optica., and a 

 thinner anterior portion the pars cceca. The line of demarcation becomes marked 

 by a thickened lip known as the ora serrata. The pars cceca becomes further divided 

 into the pars ciliaris retina where the inner layer remains as a single lamella of 

 columnar cells, and the pars iridis where it becomes closely united with the 

 pigmented outer layer, and spread over the inner surface of the developing iris, 

 to form the thickly pigmented epithelium known as the uvea. 



The pars optica -undergoes histological changes which are, in their essential and primary 

 features precisely similar to those already described for the general neural epithelium. It is at 

 first a single layer of high columnar epithelium with closely set nuclei at different levels. The 

 germinal zone is necessarily on the outer convex side of the lamella, that having been the original 

 inner surface. As the nuclei multiply a nuclear-free (or nearly free) zone is formed on 

 the concave aspect, which corresponds to the reticular zone of the general neural epithelium. 

 Sustentacular or primitive glial elements are laid down and persist as the fibres of Mutter, and an 

 outer and inner medullary lamina appear. The multiplying nuclei become arranged in zones 

 separated by narrow reticular bands. This is the expression of the grouping of the neuro- 

 blasts into radiating cell- complexes, or, interpreted by the syncytial theory, of the arrangement 

 of the neural syncytium into radiating multinuclear fibrillar paths. The nerve-fibre layer is 

 formed as elsewhere from the marginal reticular zone. The rods and cones appear first in the 

 axis of the globe as rounded refractile bodies projecting from the external medullary lamina into 

 the cleft between the two layers. They are produced progressively from the central point of the 

 retina to the periphery. Graham Kerr * finds in Lepidosiren paradoxa, in which the cells 

 are of great size, that in the elements destined to become visual cells a vesicle appears which 

 contains apparently a fatty substance. As this enlarges the cell bulges the external medullary 



1 Quart. Jour. Micro. Sci. xlvi. ; see also Cameron, Jour. Anat. and Phys. xxxix. 



