TS The Paraphysis in the Common Fowl 
the mesenchymal tissue ventrad to the forebrain, and appears as an 
irregular ring of ectodermic tissue. ‘Two of the above-mentioned ves- 
sels, which supply the choroid plexus, are met in this section. 
Fig. 5 represents a sagittal section of a ten-days’ chicken’s brain. 
The section is not exactly in the median plane, consequently the choroid 
plexus has been separated from the roof: of the third ventricle. It 
seemed to me to be unnecessary to draw in all the structures that have 
been previously represented. The epiphysis and both commissures are 
omitted. The picture, when taken as a whole, resembles very closely 
Fig. 38, an embryo of 43 mm. The dorsal wall of the posterior velar 
arch is even more perpendicular than in the above-mentioned figure. 
This tends to make the angle formed by the two limbs much more acute, 
so that now both are quite perpendicular. 
The position of the anterior arm has 
changed very little, but the choroid plexus 
springing from it, is fully developed. Un- 
fortunately, owing to the obliquity of the 
section, the velum has been separated from 
the roof of the ventricle, and consequently 
it does not appear in this section. Such a 
fold, were it present, would present no es- 
sentially different picture from that repre- 
sented in Fig. 3. As we have already seen 
the velum is situated immediately behind 
the paraphysis. JI believe that the first 
prominent fold of choroid plexus in an 
adult chicken, behind the paraphysis, repre- 
sents morphologically the large, broad, well- 
developed velum transversum of the 6.7 mm. 
embryo, as is represented in Fig. 1. There is almost no change in the 
appearance of the paraphysis. Its cavity is more constricted, but its 
walls are of about the same thickness, and of course its position in the 
two plates is identical. Above and to the left of the paraphysis there 
is a curious vesicle (Ve) which I am at a loss to explain, but will refer 
to again farther on. 
Figs. 6 and 7 represent the same sections under different powers of 
magnification. It was desirable to draw this section with the same 
power as was employed for the others, but the field was not large 
enough to include the entire section and so it was thought advisable to 
first study its topography with a lower power, without which it does not 
seem to me that it would be intelligible. The figures represent frontal 
Fria. 5. Sagittal section of a 10 
days’ chicken’s brain. XK 45.2 
diams. 
