420 



ORGAN SYSTEMS OF MAN 



Fig. 16-18. The relative development of the various parts of the brain of representative vertebrates. 



The three major regions of the brain, 

 fore-, mid-, and hindbrain, are clearly 

 marked off very early in the embryonic de- 

 velopment of every vertebrate (Fig. 16-19). 

 These three regions soon subdivide into five 

 regions. The forebrain becomes the telen- 

 cephalon and diencephalon, the midbrain 

 remains undivided and is known as the mes- 

 encephalon, and the hindbrain divides to 

 form the metencephalon and myelencepha- 



lon. Each of these becomes modified and 

 develops other parts as the brain becomes 

 more complex in higher animals. In the hu- 

 man brain the cerebrum comes from the 

 telencephalon, the posterior lobe of the 

 pituitary (an endocrine gland) and the 

 optic chiasma from the diencephalon, 

 the corpora quadrigemina from the mesen- 

 cephalon, the cerebellum and pons from 

 the metencephalon, and the medulla from 



kind brain 



midbrain 



forebrain 



telftDcepholoni 



diencephalon 



myclcDccpbalon ' meseDcepbalon' 



metencephalon JSncepbalon metiocephalpn 



mesencephalon 



myclcncepholon 



Fig. 16-19. The vertebrate brain begins as a hollov/ sac vt^hich subsequently divides first into 

 three distinct regions and later into the many parts shown (left). The embryology of the 

 human brain parallels that of other vertebrates as shovi^n by the series of figures to the 

 right. 



