58 
J. BEARD. 
described shortly, and he has allowed me to figure them in 
figs. 3, 5, 6, and 10. The three Ammocoetes of Herr Schwarz 
probably came from the same brook, and are remarkable when 
compared with every other Ammocoetes I have examined, in 
that there is a very deep deposit of black pigment in 
the parietal eye. 
Ahlborn (No. 1, p. 230) has described very correctly the 
topographical relationships of the pineal body in both Ammo- 
coetes and Petromyzon, and all I need remind the reader of 
here is that it is in both still connected with the brain, and 
not pinched off from the pineal stalk, as in Anguis, and that 
very early in development the pineal body is divided into two 
vesicles, a dorsal one, the parietal eye, and a more ventrally 
situated one, which never possesses black pigment, and never 
presents any resemblance to an eye. 
In the following account of the minute structure we are 
solely concerned with the dorsal vesicle. I shall not describe 
the structure of the ventral vesicle, which is represented as 
seen in Petromyzon (longitudinal vertical section), in figs. 8 
and 9, v. v. 
The dorsal vesicle or parietal eye ( P.E .) lies some distance 
below the surface of the body and within the skull, which dor- 
sally is only membranous (figs. 3 and 5, PI. VI). 
As pigment is at this period of the animal’s life but sparsely 
scattered in the skin there is not such a marked pigment-free 
spot above the eye indicating its position, as in Petromyzon. 
The eye is almost hemispherical in appearance, and has the 
anterior wall flattened. The anterior and posterior walls are 
separated by a narrow space (figs. 3, 5, PI. VI), which is filled 
with an albuminous coagulable fluid (fig. 9, cf.). The coagu- 
lation of this fluid led Ahlborn (No. 1, p. 233) to the conclusion 
that the cells of the two walls were connected by threads. This 
is not the case, and Spencer has already suggested the expla- 
nation given above (No. 14, p. 222, foot-note). 
The anterior wall occupies the position but lacks the struc- 
ture of a lens, as described by Spencer in Hatteria, &c., and by 
de Graaf in Anguis. It is, however, thicker in the centre than at 
