218 FREDERICK TILNEY AND LUTHER F. WARREN 



served in the ventral wall of the end-vesicle which appear to be 

 of a sensory nature. Certain large elements have been recog- 

 nized in the deeper layers of the tissue and by some authorities 

 are considered to be ganglionic cells. In addition, there are cells 

 of an ependymal nature or modifications of the latter which give 

 the impression of neuroglia tissue. There can be little question 

 that the retina of this organ is well enough defined to deserve 

 that designation. Whether it is actually functional as a visual 

 organ is not altogether clear, for the relation of the pineal eye in 

 cyclostomes to the surface of the head does not afford the most 

 advantageous conditions for a distance receptor: 



The end-vesicle of the parapineal organ closely resembles the 

 finer structure in the corresponding part of the pineal organ. 

 There are, however, certain differences which are more those of 

 degree than of kind. The rod cells, such conspicuous elements 

 in the pineal organ, are less well defined in the parapineal organ 

 and so also are the ganglionic cells. 



The differentiation of the dorsal wall of the end-vesicle in the 

 pineal as well as in the parapineal organ manifests a tendency 

 toward lens formation, for in both cases the cells in this region 

 are entirely pigment-free and give rise to a translucent structure 

 known as the pellucida. Further evidence of the visual adapta- 

 tion observed in the end-vesicle of the two structures of the 

 epiphyseal complex is the fact that the cavity of the vesicle is 

 'filled with a coagulum in the meshes of a delicate syncytium, a 

 structure which so closely resembles a primitive vitreous that it 

 may be regarded as analogous, if not homologous to that struc- 

 ture. The presence in the retina of a widely distributed white 

 pigment lends the necessary opacity to the visual membrane. 

 Both end-vesicles contain this pigment; its presence serves 

 further to convey the impression of differentiation along visual 

 lines. 



The stalks of both the pineal and parapineal organs bear a 

 certain amount of confirmatory evidence in favor of the belief 

 that the epiphyseal complex in cyclostomes has made the at- 

 tempt at visual adaptation. Nerve fibers are uniformly ob- 



