406 CHORDATA. 
Nervous System.—The drain arises as a swelling of 
the anterior portion of the dorsal nerve-tube, the posterior 
portion remaining as the spinal cord. The single swelling 
soon becomes constricted into three primary vesicles called 
the fore-brain, mid-brain, and hind-brain. The fore-brain 
then gives off the two optic vesicles as described below, and 
constricts into two secondary vesicles called the cerebrum and 
the thalamencephalon. The mid brain remains simple and 
Fig. 292.,—Four STAGES IN THE DEVELOPMENT OF THE 
VERTEBRATE BRAIN. 
Neuropore. 
< ec 
% 
2 
oQ 
= 
<o = 
, 3. 
ao 
Q 
5 
v Ps eels — 2 
: Spinal Cord. 
I,, Fore-brain. IL., Mid-brain. IIJ., Hind-brain. 
A, A tube with opening at each end. 1, Cerebrum. 
B, A swollen brain at the anterior end. 2, Thalamencephalon. 
C, Formation of the three primary vesicles. 3, Optic Lobes. 
D, Formation of the five secondary vesicles. 4, Cerebellum. 
5, Medulla. 
gives rise to the optic Jobes, and the hind-brain forms the 
cerebellum and medulla oblongata. Hence the brain has now 
five parts in succession, z.e., cerebrum, thalamencephalon, 
optic lobes, cerebellum and medulla oblongata. The 
original cavity of the brain remains to a large extent in 
these parts. The cavities in each half of the cerebrum are 
known as the /ateral ventricles, each communicating by a 
Joramen of Munro with that of the thalamencephalon or 
the ¢hird ventricle, and that of the medulla oblongata or the 
