4o6 



CHORD AT A. 



Nervous System. — The brain 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 midbrain remains simple and 



Fig. 292. — Four Stages in the Development of the 

 Vertebrate Brain. 



Neuropore. 



->5: 



\ 



I., Fore-brain. 



A, A tube with opening at each end, 



B, A swollen bram at the anterior end. 



C, Formation of the three primary vesicles. 



D, Formation of the five secondary vesicles, 



Spinal Cord, 

 II., Mid-brain. III., Hind-brain. 



1, Cerebrum. 



2, Thalamencephalon. 



3, Optic Lobes. 



4, Cerebellum. 



5, Medulla. 



gives rise to the optic lobes, and the hind-brain forms the 

 cerebellum and medulla oblongata. Hence the brain has now 

 five parts in succession, i.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 lateral ventricles, each communicating by a 

 foramen of Munro with that of the thalamencephalon or 

 the third ventricle, and that of the medulla oblongata or the 



