Franklin Dexter - 23 
Fig. 9 is a sagittal section of a 25 mm. embryo, and shows this region 
very clearly. The opening of the cavity of the epiphysis into the fore- 
brain is very apparent (Ep), and immediately anterior to it, within 
the ectodermic wall, the supra-commissure is first seen in a chick of 
about 19 mm. in length. Its position is very characteristic, for it is in- 
variably described as being situated in this particular spot throughout 
all classes of vertebrates. The posterior commissure makes its appear- 
ance at an earlier date than the superior, and is consequently at this 
period very well developed. As was previously mentioned, it is properly 
considered to be a portion of the midbrain. This being the case I 
would call attention to the fact that its fibers terminate a very short 
distance from the epiphysial opening (Ep), or in other words, imme- 
diately posterior to it, consequently, it seems to me, that the opening 
of the epiphysis into the fore-brain is situated much nearer the line of 
division between the mid- and forebrains than it is commonly supposed 
to be, and that the portion of the roof of the midbrain which is formed 
by this commissure, extends a greater distance ventrad than we are 
apt to picture in our minds. 
Some very curious, large nerve cells (N) may be observed situated in 
the wall of the midbrain. They contain a round nucleus, and an irregu- 
larly-shaped nucleolus. I call attention to them, since they are quite 
new to me. 
To recapitulate: 
1. The paraphysis appears as an evagination of the roof of the fore- 
brain, and first makes its appearance in a chick of about 6.7 mm. and 
is present throughout life. It is situated immediately dorsad to the 
foramen of Munro, is oval in shape, and contains a slit-like cavity which 
communicates with the cavity of the third ventricle. 
2. The choroid plexus of the third ventricle arises only from the an- 
terior half of the embryonic post-velar arch. 
3. In the chicken, the first prominent fold of choroid plexus posterior 
to the paraphysis corresponds morphologically to the velum transversum 
of the embryo. | 
4. The supra-commissure may be first identified in an embryo chick 
of about 19.5 mm. 
Finally, I should like to acknowledge my indebtedness to Prof. C. S. 
Minot, not only for his valuable suggestions, but also for the interest 
he has taken in this piece of work. 
