420 



THE BRAIN. 



are formed known as the fore- the mid- and the hind-brain ; of 

 these the hind-brain is usually the longest. In some instances a 

 bilobed stage can hardly be recognised. This primitive division 

 of the brain is shewn in many of the figures already given. 

 The reader may perhaps best refer to fig. 108. On the closure 

 of the medullary groove the lumen of the medullary canal 

 is continued uninterruptedly through the brain, but dilates 

 considerably in each of the cerebral vesicles. 



The anterior lobe of the brain becomes converted into the 

 cerebral hemispheres, the thalamencephalon, the primary optic 

 vesicles, and the parts connected with them. The middle lobe 

 becomes the optic lobes (corpora bigemina or corpora quadri- 

 gemina in Mammalia) and the crura cerebri ; while the posterior 

 lobe becomes converted into the cerebellum and medulla 

 oblongata. 



Before describing in detail the changes by which the primary 

 vesicles of the brain become con- 

 verted into the above parts, it will 

 be convenient to say a few words 

 about the general development of 

 the brain. 



The most striking peculiarity 

 with reference to the general develop- 

 ment of the brain is a curvature 

 which appears in its axis, known as 

 the cranial flexure. The flexure 

 takes place through the mid-brain, 

 and causes the fore-brain to be 

 gradually bent downwards so that 

 the axis of its floor forms, first, a 

 right angle with that of the hinder 

 part of the brain, and subsequently, 

 as a rule, an acute angle. 



During these changes the brain, 

 in most Amniota at any rate, becomes in the first instance 

 retort-shaped, the cerebral vesicle forming the swollen part of 

 the retort, but subsequently the retort-shape is lost owing to the 

 great development of the vesicle of the mid-brain, which forms 

 the termination of the long axis of the embryo. Figs. 29, 76, 



FIG. 247. LONGITUDINAL 

 SECTION THROUGH THE BRAIN 



OF A YOUNG PRISTIURUS EMBRYO. 



cer. commencement of the 

 cerebral hemisphere ; pn. pineal 

 gland ; In. infundibulum ; //. in- 

 growth from mouth to form the 

 pituitary body ; mb. mid-brain ; 

 cb. cerebellum ; ch. notochord ; 

 al. alimentary tract ; laa. artery 

 of mandibular arch. 



