242 HILL. [Vor. IX. 
vesicle has become its ventral wall (Fig. 11, £). Two parts 
may be distinguished, a distal enlarged portion and a proximal, 
narrow, cylindrical portion which connects the distal portion 
with the brain-roof. The distal part is flattened dorso-ventrally, 
and lies nearly in a horizontal position, with its distal end very 
close to the epidermis. Its dorsal surface is convex, while its 
ventral surface is nearly plane and is closely applied to the 
roof of the brain. The cavity of the epiphysis is reduced to 
a narrow cleft which is parallel with its dorsal surface so that 
the posterior and dorsal walls are of nearly uniform thickness. 
The ventral wall of the distal part is, on the contrary, three 
or four times as thick as the anterior wall of the proximal part 
with which it is continuous. 
The anterior epiphysial vesicle remains connected with the 
brain-roof for a period of only ten days, and shows from the 
first a less vigorous growth than the epiphysis (Fig. 12). In 
embryos 13 mm. long its connection with the brain is severed, 
and the vesicle is wedged in between the walls of the thalamen- 
cephalon and mesencephalon at some distance beneath the 
integument. It lies posterior to the superior commissure 
against the left wall of the stalk of the epiphysis. A median 
longitudinal section of the head which passes through the 
median plane of the posterior vesicle (Fig. 11) does not include 
the anterior vesicle, but a section 75 » to the left of the median 
plane bisects it (Fig. 12). It is an ovoid body the long axis 
of which is nearly perpendicular to the dorsal surface of the 
head. Its anterior and posterior walls are in close contact, so 
that its cavity is nearly obliterated. 
Posterior to the point of union of the epiphysis with the 
brain-roof is the broad posterior commissure ; and in front of 
this point is the transverse fold, which separates the prosen- 
cephalon from the thalamencephalon (Fig. 11 P and X). This 
fold is made up of a single layer of columnar cells, and projects 
into the cavity of the brain in a direction nearly parallel with 
the long axis of the epiphysis. The anterior part of the roof 
of the thalamencephalon appears, consequently, to have been 
evaginated dorso-cephalad in such a way that in a median 
longitudinal section it bears some resemblance in form to the 
