346 



THE MAINTENANCE OF THE INDIVIDUAL 



ological history. In other words, these structures are both homolo- 

 gous and in a general way analogous. 



Some of the more important changes in the growth and expansion 

 of the nerve cord are as follows. Early in its embryonic develop- 

 ment, before the five regions of the brain are developed, the anterior 

 portion of the growing nerve cord becomes differentiated into three 

 enlargements, designated, beginning anteriorly, as the iore- (prosen- 

 cephalon), mid- (mesencephalon), and hind- (rhombencephalon) brains. 



encepWlon. 



A 



prosencephalon 



mess 



/'clienogphcvloii 

 telencephalon / 

 1/ '' ^ 



■metencepholon 

 mescncepholoa v tnyelencephcJon. 



rrbombencephobn. 

 jncephalon 



Development of the vertebrate brain from a simple encephalon. 



Most of the subsequent development takes place in the fore- and 

 hind-brains (page 347). As growth continues, the anterior part of 

 the fore-brain divides, grows out into two pouchlike lateral lobes, 

 called the cerebral hemispheres (telencephalon), or collectively the 

 cerebrum. The jiosterior portion of the primitive fore-brain is now 

 designated as the 'twixt-brain (diencephalon) . The mid-brain (mesen- 

 cephalon) meanwhile remains imdivided, while the hind-brain becomes 

 separated into an anterior dorsal outgrowth, called the cerebellum 

 (metencephalon) , and a posterior medulla oblongata (myelencephalon) , 

 which is continuous with the cord. 



