04 



THE HISTORY OF CEEATION. 



Fig. 7. — Embryo of a mammal or bird, in 

 which the live brain-bladders have just com« 

 menced to develop, v. Fore brain, z. Twixt braiiu 

 m. Mid brain, h. Hind brain, n. After brain, 

 p. Spinal-marrow, a. Eye-bladders. w. Primi- 

 tive vertebrse. d. Spinal-axis or notochord. 



The first bladder, the fore brain (a), 

 is in so far the most important that 

 it principally forms the hemispheres of 

 the so-called larger brain (cerebrum), 

 that part which is the seat of the 

 higher mental activities. The more 

 these activities are developed in the 

 series of vertebrate animals, the more 

 do the two lateral halves of the fore 

 brain, or the hemispheres, grow at the 

 expense of the other bladders, and 

 overlap them in front and from above. In man, where they 

 are most strongly developed, agreeing with his higher men- 

 tal activity, they eventually almost entirely cover the other 

 parts from above (compare Plates II. and III.) The second 

 bladder, the twixt brain (z), forms that portion of the 

 brain which is called the centre of sight, and stands in 

 the closest relation to the eyes (a), which grow right and 

 left out of the fore brain in the shape of two bladders, and 

 later lie at the bottom of the twixt brain. The thir dhlsidder, 

 the noid brain (pn), for the most part vanishes in the 

 formation of the so-called four bulbs, a bossy portion of 

 the brain, which is strongly developed in reptiles and 

 birds (Fig. E, F, Plate II.), whereas in mammals it recedes 



