Franklin Dexter 19 
sections which pass through the paraphysis of a three-days’ chicken’s 
brain. In Fig. 6 one is able to distinguish the narrow lateral ventricles, 
the slit-like cavity of the forebrain lying between the two optic thalami, 
a very minute paraphysis, and just above it a triangular cavity or ves- 
icle. ‘This seems to be the same structure which we saw in sagittal 
section (Fig. 5). 
On examination of Fig. 7 the same regions are much more easily dis- 
tinguished. Here the paraphysis is seen clearly to be a portion of the 
epithelial roof of the forebrain. Its cavity presents a very uniform 
appearance, but with a higher power, small fissures are observed run- 
ning off from it, and piercing its wall for variable distances. A large. 
transverse vessel separates it from the vesicle. The space between the 
Fig. 6. Fig. 7. 
Fies. 6 and 7. Frontal sections of a 10 days’ chicken’s brain. Fig. 6 x 9.2 diams. 
Fig. 7 x 45.2 diams. 
paraphysis and the mesenchymal wall of the vessel, I imagine is due to 
shrinkage, but it must be said that it is peculiarly constant, and has 
been frequently observed as is here represented. 
Fig. 8A represents a portion of the paraphysis as seen in Fig. 5 under 
a much higher power. Even with a comparatively low power, one can 
easily subdivide its wall into two regions. An inner, thinner layer, 
next to its cavity, and an outer, much thicker layer. The inner stratum 
stains more deeply than the outer, and is of about the same thickness 
as the ependymal layer of the brain. With a high power, the nuclei of 
this layer are seen to be somewhat oval in shape, and are crowded to- 
gether in contradistinction to the outer layer where they are much 
more clearly defined, round in appearance, and more widely separated 
from each other. In a word, the outer layer seems to be a sort of modi- 
fied ectodermic tissue. The above-mentioned clefts issuing from the 
