16 The Paraphysis in the Common Fowl 
In Fig. 2, it consists of a horizontal and a perpendicular arm, which 
form together what is not 
far from a right angle. 
Moreover, it is perfectly 
apparent from this draw- 
ing, that the choroid plex- 
us of the forebrain is de- 
veloped only from the per- 
pendicular arm, or ante- 
rior portion of the arch. 
The triangular form 
presented by the velum in 
Fig. 1, has now disappear- 
ed, and it is replaced by a 
fairly thick quadrilateral 
fold of mesenchyma which 
is distinctly broader and 
more conspicuous than any 
Fig.2. Embryo of 19.5 mm. Harvard Embryological of the other folds. 
Celisetion. Sagittal series 473. 
diams. 
Section 334 
The paraphysis at this 
stage is much more developed and contains a large cavity communi- 
cating with that of the forebrain. 
the younger embryo. 
Fig. 3 is not as fortu- 
nate a median sagittal sec- 
tion. as the last... The 
paraphysis is shown ex- 
ceedingly well, but the 
communication of the cay- 
ity of the epiphysis with 
the forebrain does not ap- 
pear in this section. It is 
from an embryo of 43 
mm., and is naturally 
much more developed than 
the previous one. The 
posterior commissure is 
very large. Portions of 
the tubules of the epiphy- 
sis are seen in the mesen- 
chymal tissue above the roof of the third ventricle. 
Its wall is distinctly thicker than in 
Fig. 3. Embryo of 438 mm. Harvard Embryological 
Collection. 
diams. 
Sagittal series 509. 
Section 362. X 45.2 
The superior 
