346 



THE BRAIN. 



The anterior lobe of the brain becomes converted into the cere- 

 bral hemispheres, the thalamencephalon, the primary optic vesicles, 

 and the parts connected with them. The middle lobe becomes the 

 optic lobes (corpora bigemina or corpora quadrigemina 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 converted 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 

 development 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 form- 

 ing 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, and 118, are representative figures of embryos of various 

 vertebrate forms at a period when the mid-brain forms the termina- 

 tion of the long axis of the body. 



It is generally stated that the cranial flexure is at its maximum 

 at the stage represented in these figures, and there can be no doubt 

 that viewed from the exterior the cranial flexure ceases to be so 



marked a feature, and finally disappears 

 as the embryo gradually grows older; but 

 though the mid-brain ceases to form the 

 termmation of the long axis of the em- 

 bryo, the flexure of the brain becomes 

 in many forms absolutely more marked; 

 while in other forms, though stated to 

 diminish, it does not entirely vanish. 



The general nature of the changes 

 which take place will perhaps best be 

 understood by a comparison of figs. 247 

 and 248 representing longitudinal sec- 

 tions at two stages through the brain 

 of an embryo Elasmobranch. 'i'he actual 

 cranial flexure, i.e. flexure of the floor of 

 the brain, is obviously greater in the 

 older of the two brains, though viewed 

 from the exterior the axis of this brain 

 appears to be quite straight. In the 

 younger stage, fig. 247, the mid-brain 

 {nib) forms the end of the long axis of 



Fig. 247. Longitudinal sec- 

 tion THROUGH THE BKAIN OF A 

 YOUNG PhISTIURUS EMBRYO. 



cer. commencement of the 

 cerebral bemispbere; pn. jjineal 

 gland; In. infiindibulum; ^J*. in- 

 growth from mouth to form the 

 pituitary body; mh. mid-brain; 

 cb. cerebellum; ch. notochord; 

 al. alimentary tract; Zaa. artery 

 of mandibular arch. 



