BRAIN OF YERTEBRATA. 



503 



from a mere elongation of a shorter nerve- centre, but from the 

 gradual development of a nerve-chord, which primitively formed a 

 peripheral apparatus only. The differences in the characters of the 

 brain (exclusive of the medulla oblongata) and of the spinal chord, 

 so far as regards the arrangement of the white and gray substance, 

 serve to confirm this view, which is also supported by other facts. 



A. Central Organs of the Nervous System. 



a) Brain. 



§ 380. 



Three successive portions are developed from the rudiments of 

 the brain (Fig. 280 a), the cavities in which communicate with one 

 another. The last of these passes freely into 

 the medullary tube behind it. These primi- A 



tive cerebral vesicles give rise to new seg- 

 ments, so that we can soon distinguish five. 

 The first is known as the Fore-brain or 

 Prosencephalon (Fig. 280, a); the next as 

 the Twixt-brain or Thalamencephalon (/>); 

 the Mid-brain or Mesencephalon (B C c) 

 forms a third swelling ; and this is succeeded 

 by the Hind-brain or Metencephalon (d), 

 and the After-brain or Myelencephalon (e), 

 which is directly continuous with the spinal 

 chord, and with the metencephalon. The 

 metencephalon forms the most anterior por- 

 tion of the roof of the myelencephalon, and is 

 not therefore as distinct as the rest of the 

 cerebral vesicles. At first, the vesicles are 

 placed one behind the other, and lie in the line 

 of the longitudinal axis of the spinal chord, 

 bat they soon come to be set at an angle to 

 one another. This is due to the unequal 

 growth of the upper and lower portions, for 

 the upper ones increase greatly in size. Those 

 parts which are least developed become covered 

 over by the growth of some of the upper parts. 

 Between the prosencephalon and thalamen- 

 cephalon the wall is thinned out, and a fissure- 

 like portion developed (primitive cerebral 

 cleft, Fig. 280, a), into the interior of which a 



process from the envelopes of the brain is continued. This is not a 

 time lacuna, but is merely due to the gradual thinning-out of the 

 wall of this portion. The epiphysis (pineal gland) is developed 

 from a part of this roof. 



The lower portion of the thalamencephalon forms the floor of the 



Fig. 280. Vertical aud 

 median sections through 

 a Vertebrate brain. A Of 

 a young Selachian 

 (Heptanchus). B Of the 

 embryo of an A d d e r. 

 C Of the embryo of a 

 Goat, a Prosencephalon. 

 l> Thalamencephalon. c 

 Mesencephalon (in A it is 

 marked by d). d Meten- 

 cephalon. e Myelence- 

 phalon. s Primitive cere- 

 bral cleft, h Hypophysis. 



