THE central nervous system is formed in the foetus by an infolding 

 of the epiblastic layers. At the anterior end of the nervous tube 

 thus formed the brain becomes developed. This is the end of the 

 animal which normally progresses forwards, and it is here, therefore, 

 that develop the special sense mechanisms which serve -to protect 

 the animal. This end of the neural tube becomes thickened, and 



rudiment of cerebellum 

 inf. med. uef. 



ceph. flex. 



posit of 



dor. sellae. 

 optic uesic. 



ligula 



obex 

 nuchal flex. 



olf. lobe 



rest/form body 



FIG. 411. LATERAL VIEW OF CEPHALIC PART OF NEURAL TUBE IN A FIFTH-WEEK 

 HUMAK EMBRYO. (Keith, after His.) 



enlarges to form three primary cerebral vesicles the prosencephalon, 

 mesencephalon, and rhombencephalon. The first and third of these 

 again divide, so that we have eventually five primary parts: 



First primary vesicle : 



Second primary vesicle . 

 Third primary vesicle : 



\ 



The pros-encephalon (fore-brain). 

 The di-encephalon, or thalam- 

 encephalon (between-brairi). 

 The mes-encephalon (mid-brain). 

 The met-encephalon (hind-brain). 

 The myel-encephalon (after-brain). 



The myelencephalon becomes developed, to form the spinal bulb. 

 The bulb connects the spinal cord with the rest of the brain. 



From the metencephalon develops the little brain, or cerebellum, 



685 



