240 HILL. [Vou. IX. 
the vesicle, and opens into the common passage which leads 
from the brain cavity. This passage, which is median, rises 
for a distance of .0o21 mm. and then bifurcates, one branch 
passing to the right to form the cavity of the posterior vesicle 
and the other to the left to form the cavity of the anterior 
vesicle (Fig. 4). 
As may be seen in a transverse section the left wall of 
the posterior vesicle passes directly into the right or dorsal 
wall of the anterior vesicle, while the right wall of the posterior 
vesicle and the left wall of the anterior vesicle are each con- 
tinuous laterally with the brain-roof (Fig. 4). Thus the two 
vesicles are attached to the brain-roof by a common stalk. 
These vesicles may have been formed in any one of three 
ways: (1) They may be thought of as independent and separate 
outgrowths which have subsequently come to be borne on a 
common evagination of the brain-roof; (2) it is possibie to 
think of them as formed by the division of an originally single 
vesicle ; (3) it is possible to think of the anterior one as formed 
by constriction from the posterior. It was, perhaps, under the 
influence of the latter point of view that Strahl and Martin 
and Béraneck have described the anterior vesicle of Lacertilia 
as formed at the expense of the posterior. I have not obtained 
satisfactory sections of embryos younger than that which 
yielded the section shown in Fig. 4, and if one judges by this 
figure alone there is little to choose between the three methods 
of origin mentioned above. In Fig. 3a there is shown a view 
of the roof of the fore-brain in an embryo thirty-seven days 
old, and in this the two epiphysial vesicles appear as slight and 
entirely independent elevations of the outer surface of the brain- 
wall. Beneath each of these elevations is seen a slit-like lumen 
leading from the brain-cavity toward the outer surface of 
the brain-wall. These lumina are entirely separate from one 
another. Unfortunately this observation was made on but 
one living individual and, owing probably to the rapidity 
with which the stage is passed through, I was unable to 
verify it on others. 
The brain-roof at the point of union with the stalk of the 
two vesicles is very much thinner than the adjacent brain- 
