NERVOUS SYSTEM. 



49 



and hind brains. That these three regions are not exactly com- 

 parable to the segments of the spinal cord is shown by the fact 

 that the same neurorheres characteristic of the cord (p. 46) 

 appear in the mid and hind brains. Each of these vesicles 

 contains an enlarged portion (called primary ventricle) of that 

 cavity, which in the cord is called the central canal. 



B 



FIG. 50. Diagrams of the devel- 

 opment of the brain. In A the three 

 primary vesicles; in B the differentia- 

 tion of the definitive regions. C, 

 cerebrum ; CB, cerebellum ; F, fore 

 brain; H, hind brain; Z, lamina 

 terminalis; M, mid brain; MO, 

 medulla oblongata ; S, spinal cord. 



FIG. 5 1 . Section through 

 the brain of embryo pig, 6.5 

 mm. long, showing the seg- 

 mentation (neuromeres) of 

 the hind brain (M). o, otic 

 capsule ; 7 & 8, facial and 

 auditory nerve ; 9, glosso- 

 pharyngeal nerve. 



Soon these three vesicles become differentiated by unequal 

 growth into five regions, the fore and hind brain each giving 

 rise to two, the mid brain remaining unaltered. 



From the fore brain arise the prosencephalon (telencephalon, 

 cerebrum or cerebral hemispheres) and the thalamencephalon 

 (optic thalami, diencephalon or twixt brain) in the following 

 manner. The extreme tip of the fore brain in the median plane 

 remains stationary, and forms a thin membrane, later known as 



