RUDIMENT OF THE BRAIN. 34 



o 



shaped or lyre-sliaped germ-sliield (Figs. 86, 87, p. 298). In 

 the middle line of its dorsal surface the primitive groove 

 first made its appearance, enclosed b}" the two parallel 

 dorsal, or medullary swellings. The coalescence of these 

 formed the medullary tube. When we examine the further 

 modifications of this, we very soon perceive a difference 

 between the formation of the anterior and that of the 

 posterior ends. At the anterior end in Man, as in all the 

 higher Vertebrates, the brain very soon begins to separate 

 or diflferentiate from the medullary tube. The first rudi- 

 ment of the brain is merely a roundish, bladder-like pro- 

 tuberance of the vertebral canal (Fig. 103, lib). Very soon, 

 however, this bladder is divided by two circular contrac- 

 tions of its circumference, into three consecutive vesicles, 

 the so-called primitive brain-bladders (Fig. 104, v m h). 

 Two other similar contractions then appear, so that we now 

 find five brain-bladders in a row (Fig. 105). This is the 

 mode of development of the brain in all Mammals, from the 

 simplest Fishes to Man. In all, we find a simple vesicle as 

 the first rudiment of the brain, which is afterwards parted, 

 by contractions in its circumference, into ^lyq smaller 

 bladders. Though the brain, as the organ of the soul and 

 the mental activities, afterwards develops in various Verte- 

 brates in such very various ways, yet the first rudiment 

 is of this simple and homogeneous form. This is a fact of 

 the highest importance. 



Directly below the medullary tube, in the lyre-shaped 

 germ-shield, we found the notochord. Right and left of the 

 notochord the two parallel primitive vertebral cords had 

 split away from the side- layers. But while the five brain- 

 bladders are becoming distinct at the anterior end of the 



