70 
J. BEAED. 
And another instance is shown in the ontogenetic and 
phylogenetic development of all the complicated parts of the 
auditory organ from one bit of sensory epithelium. This 
development of subsidiary sense organs from one piece of 
parent sensory epithelium is a most remarkable fact of 
embryology to which I hope to draw the attention it deserves. 
Distribution of the Parietal Eye. 
These researches show that the parietal eye was developed 
in the group of Fishes, and still has the characteristics of an 
eye in the very primitive group of Cyclostomata. It is not 
impossible that in other fishes it may still present a good 
development, though it is not very likely that such will be 
found to be the case in any existing Elasmobranchii and 
Ganoids. 
In this connection a figure in ZitteFs ‘ Paheontologie ’ 1 seems 
to me very interesting. It represents the bony skeleton of a 
Placoderm Ganoid, Asterolepis ornatus, from the Old Red 
Sandstone, and on the dorsal surface in the centre of a bone, 
marked os dubium, there is something which looks suspiciously 
like a parietal foramen. 
Why the eye has degenerated can hardly yet be determined. 
No doubt it has suffered in its competition with the paired 
eyes. Apparently, too, it was worse fitted out with accessory 
structures such as muscles, &c., than these. 
I shall not attempt to discuss the question of whether it 
is still functional or not in Cyclostomata. As no lens is 
developed there it can be of little use as an organ of vision, 
while I think Wiedersheim (No. 17, p. 149) has made out a 
good case for its functional use in such forms as Hatteria. In 
Cyclostomata it has all the characteristics of a degenerate 
organ, one especially in a very high degree, viz. its variability 
1 Zittel, * Handbuch der Palseontologie.’ Abtheilung I. “ Palseozoologie,” 
Bd. iii, Heft. 1, tig. 161, p. 155. 
