CHAP. XIX.] DEVELOPMENT OF THE BRAIN, 833 



The mode of origin of, as well as the early changes 

 taking place in, these three nervous vesicles are essentially 

 similar, up to certain stages, throughout the Vertebrata. 

 From such a basis, common to all, the several types of 

 Vertebrate Brain are developed. Our attention must, how- 

 ever, now be confined to tracing in brief outline the mode 

 in which the brain of Man gradually develops from the 

 simple stages common to it and the Vertebrata generally. 



In order that the reader's attention may be more effectively 

 concentrated upon the subsequent changes undergone by the three 

 swellings of the primary nerve tube, it may be well here to anticipate 

 a little, and state what are the several parts of the Brain which 

 gradually develop from them or their derivatives. 



The Posterior Swelling (or Hind-brain) becomes divided into two 

 regions, of which the hindermost corresponds with what subse- 

 quently develops into the posterior half of the Medulla, and here 

 in the situation of the Fourth Ventricle the upper wall of the tube 

 becomes thinned away till all nervous matter disappears, and only 

 a mere membrane remains (' pia mater ') to roof over the space 

 above mentioned, which is continuous with the central canal of the 

 tube behind it. The anterior region of this swelling corresponds 

 with the anterior half of the Medulla. From the back or sides of it 

 there grows a distinct segment of the future brain, viz., the Cere- 

 bellum (fig. 122, c &). Much later on, when the lateral lobes of 

 the Cerebellum have made their appearance, this region of the 

 Medulla is crossed below by the Pons Varolii (p). 



The Middle Swelling (or Mid-brain) affords the matrix from 

 the upper part of which develops the Optic Lobes or Corpora 

 Quadrigemina (fig. 122, q). From the lower part are differentiated 

 prolongations from the fibrous ' columns ' of the Cord and Medulla, 

 known as the Cerebral Peduncles (r). The cavity within this swell- 

 ing gradually diminishes till in Man it persists only as a narrow 

 passage (6) between the cavities of the Hind and of the Fore-brain 

 (the Fourth and Third Ventricles) (a c). This passage is known 

 as the ' Sylvian aqueduct.' 



The Anterior Swelling (or Fore-brain) undergoes remarkable 

 modifications, principally on account of certain extraordinary out- 

 growths to which it gives rise. From the sides of this swelling 



