32 Journal of Comparative Neurology. 



eye and showed that the arrangement of the rods and nuclei 

 and cells of the retina of the parietal eye is essentially that of 

 the same elements in the more perfect organs of Hatteria 

 and Varanus, as described by Spencer. As in the adult the 

 parietal eye of Ammoccetes is a variable organ — a point which 

 is naturally of importance in connection with the question of its 

 degeneration. 



In Petromyzon planeri the external position of the organ 

 is marked by a large whitish spot on the skin behind the olfac- 

 tory hypophysial opening. In Petromyzon marinus it is es- 

 pecially large and, as in Petromyzon fluviatilis, the spot is due 

 to the absence of black pigment over that portion of the skin. 

 Corresponding to this white spot there is a depression in the 

 cranium just beneath it. As this depression in P. fluviatilis is 

 always associated with a fair development of the parietal eye in 

 the individual, Beard assumes that the organ will probably be 

 found well developed in the marine form as it is less degener- 

 ated than the fresh-water one. The same variation in the pres- 

 ence or absence of pigment found in Ammocoetes occurs also 

 in fresh-water Petromyzon. Relatively to the brain the organ 

 in the adult lies further forwards, and is connected throughout 

 life with the brain by a somewhat long stalk. The parietal eye 

 in the adult usually lies in a deep depression of the skull, but 

 if no pigment be present in the eye, the corresponding depres- 

 sion in the skull is almost or entirely absent. The amount of 

 pigment deposited in the eye varies in different specimens. The 

 pigment referred to above is black. Beard did not find the 

 white pigment seen by Ahlborn, nor the grey-white noticed by 

 Wiedersheim, and therefore concluded that it was dissolved out 

 in preparing his sections. In Petromyzon and Ammocoetes 

 alike, the cavity of the vesicle is filled by a coagulable fluid. 

 The posterior walls of the vesicle, or retina is made up of three 

 layers : an inner layer of rods, which also contains pigment ; a 

 layer of nuclei; and outside a granular striated layer containing a 

 few ganglion-cells. Connective-tissue invests the whole eye. The 

 end elements of the retina are of two kinds comparable to those 

 in the retina of the ordinary eyes. 



