No. 2.] EPIPHYSIS OF TELEOSTS AND AMIA. 241 
wall, so that the two epiphysial evaginations rest in a shallow 
depression of this roof. The epithelium dorsal to the vesicles 
consists of a single layer of flattened cells (Fig. 4). Between 
this layer and the brain mesenchyme is always present except 
just dorsal to the epiphysial vesicles. A section through the 
epiphysial vesicles parallel to a plane through the middle of 
the lens of the left eye and through the posterior border of 
the lens of the right eye passes through the cavities of both 
vesicles (Fig. 4). Such a section makes an angle of ten 
degrees with a transverse line. 
At a distance of .184 mm. in front of the passage common 
to the two vesicles there is a small transverse fold of the brain- 
roof which extends in a postero-ventral direction into the cavity 
of the primary fore-brain in such a way that the brain-roof 
between it and the epiphysial vesicles has the appearance of 
having been evaginated in a dorso-cephalic direction (Fig. 2). 
Rabl-Riickhard has indicated this fold as the boundary between 
the prosencephalon and the thalamencephalon. Again at a 
distance .122 mm. caudad of this passage there is a similar 
fold accompanied by a thickening of the brain-roof. This is 
the beginning of the posterior commissure. 
The two epiphysial vesicles are encircled by a blood-vessel 
which passes transversely across the roof of the brain and in 
which the blood in some embryos circulates from right to left, 
and in others from left to right (Fig. 3). 
The histological structure of the two epiphysial vesicles at 
this stage of development is essentially the same as that of 
the adjacent brain-roof. The nuclei are ovoid and so arranged 
that their long axes are radial to the cavity. The walls of the 
vesicles are usually two cells deep, except the lateral walls of 
the posterior one, which are more than two cells deep (Fig. 4). 
Since the posterior vesicle becomes the epiphysis of other 
writers, I shall in the following stages speak of it as such. 
Salmo 13 mm. long (92 days old).—The epiphysis of Salmo 
13 mm. long has grown forward. Its median plane coincides 
with the median plane of the body. Its long axis is curved 
and its distal portion approaches to parallelism with the axis 
of the body, so that what was formerly the anterior wall of the 
