THE NERVOUS SYSTEM. 71 
organ in a mammal may be explained by reference to the general 
plan as indicated in Fig. 39, which is based upon general features 
of form in vertebrates and upon embryonic development. For 
comparison in the gross the brain of the frog (Fig. 38) offers one 
of the best examples. 
The brain as first formed in the embryo appears as an anterior 
expanded portion of the neural tube, or rather as three expansions 
arranged in a linear series. They are described as the primary 
cerebral vesicles; or, as primary divisions of the future brain, 
they are designated in anteroposterior order as 
the prosencephalon, mesencephalon, and 
rhombencephalon. 
The first of the primary divisions, the pro- 
sencephalon, or primary fore-brain, becomes 
divided during development into two portions, 
namely, an anterior portioa, the end-brain or 
telencephalon, which is largely a paired struc- 
ture, and a second portion, unpaired, the dience- 
phalon, or inter-brain. The larger paired 
portion of the telencephalon is the basis of the 
cerebral hemispheres. It contains, as divisions eer ea 
of the primary cavity, a pair of cavities, the of the frog from 
the dorsal surface. 
lateral ventricles. The anterior portion of the ©, cerebellum; 4, 
diencephalon; fv, 
telencephalon, moreover, becomes differentiated, fourth ventricle; h, 
cerebral hemisphere 
so that a small terminal olfactory segment, the Oe LS: 
rhinencephalon, is more or less _ perfectly 
marked off from the rest. In the mammalian brain this part 
is chiefly identifiable as the paired olfactory bulb, the latter 
being the anterior portion of the olfactory lobe or olfactory brain, 
and containing in its interior an extension of the lateral ventricle. 
The unpaired portion of the prosencephalon is considered as 
belonging in part to the telencephalon and in part to the dience- 
phalon. Its cavity, the third ventricle, is connected with the 
lateral ventricles through the interventricular foramen. Its 
anterior wall is formed by a transverse connection of the cerebral 
hemispheres, the lamina terminalis. In all vertebrates this 
portion of the brain is remarkable for the manner in which its wall 
is differentiated. The ventral portion extends downward as a 

