60 
J. BEARD. 
prepared according to the usual methods, viz. sublimate or even 
chromic-osmic-acetic acid, to make certain of the connection 
of the rod-like retinal elements with the inner and outer nuclei. 
As the demonstration of such connection is more the work of 
the histologist than of the morphologist, and as I am concerned 
here more with the morphological aspect of the matter, I am 
content to have shown (1) the presence of black pigment in the 
parietal eyes of some Ammocoetes, and (2) that the arrange- 
ment of the rods and nuclei aud cells of the retina of the 
Ammocoetes parietal eye is essentially that of the same elements 
iu the more perfect organs of Hatteria and Varanus, as described 
by Spencer. 
As iu the adult the parietal eye of Ammocoetes is a variable 
organ — a point which is naturally of importance in connection 
with the question of its degeneration. 
The Parietal Eye in Adult Petromyzon. 
In Petromyzon planeri I have been able to make a fairly 
exhaustive investigation of the organ iu question. Petro- 
myzon marinus only came into my hands in the shape of one 
specimen in a bad condition for histological work. For this 
specimen, aud for an example of Bdellostoma, to he afterwards 
mentioned, I have to thank Professor Howes. 
Externally the position of the organ is marked by a large 
whitish spot on the skin behind the olfactory hypophysial open- 
ing. Iu Petromyzon marinus it is especially large, and, as 
in Petromyzon fluviatilis, this spot is due to the absence 
of black pigment over that portion of the skin. If this white 
spot is any criterion for the existence of the parietal eye in a 
fully-developed condition, as I believe is the case, then Petro- 
myzon marinus and Mordacia mordax, as figured by 
Gunther (No. 9, p. G93, fig. 318), must both possess the 
parietal eye in a fair state of development. The latter form, 
which is very rare, has not been at my disposal, and owing to 
the condition of preservation of the specimen of Petromyzon 
marinus I can give little information of the state of the 
