178 W. BALDWIN SPENCER. 



parable in the two cases^ being in the Invertebrates mentioned 

 formed as a cuticular secretion. 



In Hatteria as in all forms examined it is distinctly cellular, 

 the nuclei being prominent and numerous (fig. 2). The 

 median cells are elongate so as to give the lens a curious cone 

 shape, the base corresponding to the front of the eye and the 

 apex lying in the optic axis ; the cells are further arranged in 

 a definite manner as shown in fig. 2, and are, as the latter 

 indicates, directly continuous with those of the retina. 



(2) Retina. — The retinal elements are arranged in the 

 manner typical of Invertebrates, i. e. the rods lie on the inner 

 side bounding the cavity of the optic vesicle, the nerve enter- 

 ing posteriorly and not spreading out in front of the rods. 



Within the same vertebrate animal we thus find 

 eyes developed on both vertebrate and invertebrate 

 types, both being also formed fromthe modification 

 of the walls of hollow outgrowths of the brain. 



The retina consists of the following elements (PI. XIV, figs. 

 2, 3, 4, and 5) : 



(1) A layer of rod-like bodies {R) enveloped in deep pig- 

 ment, which when the rods are separated (fig. 5) is seen to be 

 so deposited upon them as to produce a striated appearance. 

 The pigment is specially densely deposited around the margin 

 of the retina in contact with the lens, extending here through 

 the whole thickness of the wall. A curious specialisation 

 takes place in connection with the rods lying in the optic axis, 

 which also obtains in the pineal eye of many other forms. 

 The rods in this portion are elongated (R^) to at least twice the 

 length of the ordinary ones, and are in connection at their 



■^iu^\-,\-.od. outer ends with a special group of nucleated cells (n^) which 

 lie enclosed by a somewhat definite membrane in the pineal 

 stalk, with the fibres of which they are directly connected 

 (fig. 4). 



(2) A double and, in parts, triple row of spherical nucleated 

 elements (n}), which appear to be connected by processes, on 

 the one hand with the rods, and on the other with the layers 

 external to them. They surround posteriorly the elongate rods, 



