80 CASWELL GRAVE 



the two types of cells in the eye of either mature or immature 

 tadpoles of Amaroucium, and my observations are therefore 

 in agreement with those of Kowalewsky ('71), who found that 

 the pigment granules lie within the inner ends of the visual 

 cells of the retina. Cell walls are nowhere definite in the nerve 

 tissues of the Amaroucium tadpole, however, and it is possible 

 that the pigment granules are formed in cells distinct from those 

 in which the visual rods are developed. Studies of the eyes of 

 the larvae of other ascidians now in progress may clear up this 

 point. 



No migration of pigment granules within the retinal cells was 

 observed in the living tadpole, and sections of the eyes of tad- 

 poles fixed in Flemming's solution after an exposure of thirty min- 

 utes in the dark showed no observable difference in the distri- 

 bution of the pigment granules from that of tadpoles exposed to 

 strong light before similar fixation. 



The lenses of the ascidian eye have been described as modified 

 or transformed cells, but in Amaroucium I find they are not 

 modified cells, but are deposition products formed within vacuoles 

 of marginal cells of the retinal ganglion, the nuclei of which, 

 in the embryo, are similar in size and structure to the nuclei of 

 the adjacent nerve cells of the retina (fig. 4). The portion of 

 each lens-forming cell which projects into the cavity of the sen- 

 sory vesicle is greatly enlarged and contains a large vacuole, 

 in the center of which a transparent spherical droplet of amber- 

 colored substance is deposited. In the earliest stage noted the 

 size of the droplet of lens substance was small. Other stages 

 showed that the lenses gradually increase in volume until they 

 entirely fill the vacuoles. Some cases were noted in which more 

 than one droplet of lens substance were present in the same vacu- 

 ole, a fact which indicates that the substance is semiliquid in 

 nature, for the droplets presumably flow together to form a single 

 lens. The illustrations which show the lens-forming cells and 

 the lenses (figs. D, 4, 5, 6, and 11) are misleading if they convey 

 the idea that the lenses are lamellated or have a concentric 

 structure. They are homogeneous in appearance both in the 

 Hving tadpole and in sections. At the center of each lens, how- 



