BY w. J. McKay. 883 



layer (jig. 6, N""). Thus there is produced such an appearance, as 

 De Graaf has described in Anguis fragilis, of a row of rods supe- 

 rior to the pigmented rods. This may be the explanation of De 

 Graaf's figures, which Spencer has objected to. The pigment is 

 deposited in the rods as minute particles in horizontal lines ; while 

 it seems to be in vertical lines in the body of the retina. 



The histology of the retina cannot be so well made out now as 

 before ; but the rods, some of the sphei-ical elements, the lower 

 layer of spindle-shaped bodies, the clear area, and the lowest layer 

 of nuclear elements can all be defined by examining a series of 

 sections. Why the difie^-^nt elements cannot be so clearly made 

 out, seems to be that the retinal elements are more closely applied 

 one to another. 



Third Stage. — The chief thing to be noticed in this (fig. 13), the 

 last stage that I have, is the further development of the pigment, 

 which now covers the whole of the rods in many places ; and where 

 the lens joins the retina, the pigment reaches through nearly the 

 entire thickness of the -retina. Another point of importance is 

 that ])igment is developed in the lens. It extends as a band at 

 the periphery of the lens. Above the eye the epidermis has 

 become more marked, and is now represented by an external 

 or cuticular layer, a lower and thicker layer which appears 

 at first sight to be composed of wavy fibres, this being the 

 scales in process of formation, and a third layer of columnar 

 cells, the rete mucosum. All these layers are continuous over 

 the eye. 



In compaj'ing the alteration in shape in the eye in its three 

 stages, we see (by referring to figs. 11, 12, 13), that it is always 

 double-convex in outline, but that the lens changes from concavo- 

 convex to double-convex, and that consequently the optic vesicle 

 presents at first a double convex outline in section, but that this 

 alters to concavo-convex. And along with this change in shape of 

 the optic vesicle, its vertical height deci'eases until it is almost nil. 

 The chief factors in this obliteration of the optic vesicle are — (1). 

 57 



