THE NERVOUS SYSTEM AND THE EYES. 15 



be recognised, and each of these portions corresponds to the rudi- 

 ment of one of the eyes. 



The dorso-ventral flattening undergone by the common optic pit 

 considerably diminishes the size of its cavity, of which, finally, only 

 traces can be found (Fig. 10, A). The external aperture also closes 

 completely. The thickened upper wall of the optic pit is now in 

 close contact with the hypodermis (Fig. 10, A, r), and pigment has 

 already appeared in it. It represents the retina of the eye, and, later, 

 by gradual differentiation, yields the groups of retinulae, as well as 

 the pigment-cells between them. The hypodermal layer (h) which 

 lies over the retina becomes the vitreous body, and secretes the lens 

 externally ; -it has therefore recently been designated as the lentigen 

 layer (Mark). The cell-layer lying behind the retina, i.e. the lower 

 wall of the optic pit (Fig. 10, pr), is the post-retinal layer of ecto- 

 derm-cells found in the adult. This layer secretes posteriorly the 

 cuticle which surrounds the optic cup (Fig. 10, G). The post-retinal 

 layer itself comes into close contact with the retina at a later stage. 

 A cuticle resembling the basal membrane of the post-retinal layer 

 also appears between the cells of the vitreous body and the retina 

 (Parker). It represents the fused cuticular borders of these two 

 cell-layers and separates them from one another (Fig. 10, O). 



The innervation of the developing eye is of special interest. It 

 has already been shown that part of each cephalic pit enters into the 

 formation of the brain, and the optic ganglia must, indeed, arise 

 chiefly in this way. In Fig. 12, B and C, these ganglia are seen 

 connected with the optic pit. Later, they become almost entirely 

 separated from it, retaining only a narrow and drawn-out connection 

 representing the optic nerve (Fig. 12, C and D). On the inner 

 side the optic ganglia are connected from an earlier stage with the 

 brain (Fig. 12, A and D). 



In the earlier stages in the development of the eye, the optic 

 nerve is at first connected with the convex surface of the optic 

 invagination (Parker), and the nerve fibres seem chiefly to unite 

 with the surface which is turned towards the hypodermis (Fig. 10, 

 B). This surface, however, corresponds to the side of the retina 

 which, in the adult eye, is directed towards the exterior, i.e. the 

 nerve-fibres at this stage unite with those ends of the retina-cells 

 which, in the adult eye, are the free ends, and are directed outward. 

 Directly opposite conditions are thus found in the embryo and in the 

 adult, and it must be assumed that the nerve-endings shift, during 

 the course of development, from the outer to the inner ends of the 



