118 THE FEOG. 



whole central nervous system may be compared to a retort (Fig. 

 60), the bulb of the retort being formed by the anterior and 

 vertical part of the brain, BF, and the neck by the posterior 

 horizontal part of the brain, together with the spinal cord. 



This bending of the brain is spoken of as cranial flexure. It 

 takes place, as shown in Fig. 60, round the anterior end of 

 the notochord, and is due, in the first instance, to the spherical 

 shape of the surface of the egg on which the neural plate is 

 formed. A similar ventral flexure of the hinder end of the 

 neural tube is present at first, but becomes early obliterated by 

 the outgrowth of the tail (cf. Fig. 58, B, C, D, E). The ventral 

 flexure of the brain, round the anterior end of the notochord, 

 persists throughout life. 



Very shortly after the closure of the brain-tube is completed, 

 a slight transverse constriction appears, at the bend between 

 the horizontal and vertical portions of the brain, and a little 

 later a second constriction is formed rather further forwards. 

 By these constrictions the brain (Figs. 60 and 61) becomes 

 divided into three portions, named fore-brain, mid-brain, and 

 hind-brain respectively. 



The fore-brain (Figs. 60 and 61, BF) is the terminal ver- 

 tical portion, corresponding to the bulb of the retort ; the mid- 

 brain. BM, which is the smallest of the three divisions, forms the 

 angle of the bend, opposite the anterior end of the notochord ; 

 and the hind-brain, BH, is the horizontal portion, continuous 

 posteriorly with the spinal cord. This division of the embryonic 

 brain into three regions, anterior, middle, and posterior, is a 

 convenient one, as it obtains throughout the higher groups of 

 Vertebrates, from fishes to mammals, each of the divisions 

 giving rise to important and characteristic parts of the adult 

 brain. 



The walls of the brain-tube are at first of approximately 

 uniform thickness in all parts, excepting the roof of the hind- 

 brain, which from the first is thin. By unequal thickening of 

 various parts, especially of .the sides, and by outgrowths, either 

 median or paired, with accompanying histological changes, the 

 adult brain is gradually built up. 



In these changes the most important share is taken by the 

 fore-brain. The fore-brain itself becomes the part known in 

 the adult as the thalainencephalon, its cavity persisting as the 



