MICROSCOPICAL STUDIES. 69 



attained in the eye of the Limpet ; the epidermis at one spot, having 

 sunk down to form a shallow depression wherein the cells forming 

 the floor of the cavity constitute a primitive retina. 



In Fig. III., the edges of the optical pit have grown horizontally 

 inwards so as to reduce the mouth of the pit to a small round open- 

 ing. This pit raised up on a stalk marks the permanent condition of 

 the eye in Nautilus (Fig. IX.). In the embryo Sepia, the aperture is 

 early obliterated by the approximation of the free edge of the epi- 

 dermal fold. This accomplished, we have a hollow globe formed 

 overlaid by a continuous epidermal layer (e, Fig. IV.) A condition 

 of eye exactly corresponding to this stage, is the adult form of eye of 

 Helix as described above. 



By a second downgrowth of the surface epidermis, another pit- 

 like cavity is formed, the floor of which impinges upon and fuses 

 with the anterior wall of the previously formed hollow ocular sphere. 

 Fig. V. illustrates this stage. At the centre of the area where the two 

 layers fuse, a transparent nearly spherical body, the future lens, 

 begins to form. According to Carriere the external of the two fused 

 layers forms the external part, whereas the anterior wall of the optic 

 sphere forms the larger internal half. Hence the plane of division 

 that cuts the mature lens into two parts represents two fused epi- 

 dermal layers, and the fibrous strands of which it is composed merge 

 equatorially into the surrounding ciliary body. The lens itself is 

 composed of structureless layers, that are however only revealed 

 after treatment with chemical reagents. Naturally, it is transparent, 

 tough, semi-gelatinous, and apparently homogeneous. The folds in 

 front of lens (e) represent the origin of the ultimate iris ; the cornea 

 is likewise formed by another fold of the epidermis turning inwards 

 and fusing in the same manner as the layer e in figs. III. and IV. 



The mature form of the eye is now reached, the posterior wall of 

 the ocular sphere becoming modified into the retinal layer by 

 differentiation of the cells. Comparing the development of the 

 vertebrate eye, we find the retina is there formed, not directly from 

 an epidermal invagination, but as an outgrowth from one of the 

 primitive vesicles of the brain (Fig. VII.), which eventually assume 

 the form of a double walled stalked cup, through the external wall 

 of the outgrowth becoming pushed in upon itself, while the hollow 

 stalk comes finally to represent the optic nerve. Synchronizing with 

 these changes, an epidermal invagination has been taking place, 

 which by ingrowth of the lips is at first a closed sac connected at one 

 spot with the overlying epidermis, but which is quickly severed and 

 sinking inwards, is subsequently converted into a transparent lens, 

 filling the mouth of the retinal cup. The cornea here, as in the eye 

 of Sepia, is formed by the epidermal layer that overlies tha lens 

 losing its cellular nature and becoming transparent and colourless. 



It will be seen from the foregoing that in Sepia, except for the 

 nerve fibres that surround and penetrate the retina, and for the 



