56 
J. BEARD. 
The circumstances which led both these observers to the hypo- 
thesis, afterwards converted into a fact by de Graaf and 
Spencer, did not favour Ahlborn with the discovery of black 
pigment in the pineal body of Ammocoetes. Had he chanced 
to obtain sections such as I figure in figs. 1, 3 and 8 of 
PI. YI, there can be little doubt that he would rightly have 
regarded his idea of the rudimentary eye-nature of the pineal 
body as more a fact than an hypothesis. I can, from my own 
researches, easily understand Wiedersheim’s failure to find a 
coloured pigment, black or otherwise, in the pineal body of 
Ammocoetes, for I have only seen it in the three Ammocoetes 
to be afterwards described, and such pigment appears to 
occur very rarely in the parietal eye in the Ammocoetes stage. 
But for a long time Ahlborn’s failure was to me an enigma 
which I could only explain on the supposition that he had 
never had fully adult Petromvzon in his hands, for I was 
fortunate enough to find the black pigment in the first full- 
grown Petromyzon examined, and it was some time before a 
Petromyzon in which the pigment was absent came into my 
hands. 
Wiedersheim’s and Ahlborn's negative results regarding 
the presence of black pigment are easily explicable, as will 
afterwards be seen. Not all Petromyzon, and still less Ammo- 
coetes, possess the black pigment in the parietal eye. 
Since Spencer’s researches appeared, the only accounts of 
actual work published on the parietal eye are the preliminary 
notice of my own discoveries (No. 3, p. 246) and Beraueck’s 
account of its development in Lacerta and Anguis (No. 4). 
I shall have occasion further on to refer to Beraneck’s paper. 
Here, be it remarked, that in his account of the development 
there is little or nothing that was not already known. 
I have also worked the development in these two forms, and 
if I refrain from publishing the results it is only because I agree 
with another observer, who also has investigated the matter, 
that there is little or nothing to make known which is not 
already common knowledge. 
The Cyclostomata were chosen for the following research on 
