SXTBYEY OF HUKAN NEBYOUS SYSTEM. 33 1 



m. Third Period : Medulla of Qhordonia. 



The central nerve system consists of a simple mednllarj tnbe, an 

 elongation of the upper throat ganglion, which ia separated from thn inteo- 

 tine by a notoohord (chorda dorsaUs). 



lY. Fourth Period : Medulla ofAerania. 



The simple medullary tube differentiates into two parts : a head, and a 

 cLsfTsal part. The head medulla resembles a small, pear-shaped, simple 

 swelling (the primitive brain, or first rudiment of the brain) on the anterior 

 oztremity of the long cylindrical spinal marrow. 



Y. Fifth Period : Medulla of CyclostonM, 



The simple, bladder-like radiment of the brain divides into flre oon- 

 ■eontiTe brain-bladders of simple straotnre. 



YI. SitBth Period : Medulla of Primitive Fishes, 



The five brain-bladders differentiate into a form similar to that now 

 permanently retained by the Selachii. 



YII. Seventh Period : Medulla of Amphibia, 



The differentiation of the five brain-bladders progresses to that stmotnre 

 which is now characteristic of the brain in Amphibia. 



^ Yin. Eighth Period : Medulla of Mammals. 



The brain attains the characteristic peculiarities distinctive of Mammals. 

 The following may be distinguished as subordinate stages of development ; 

 1, the brain of Monotremes ; 2, the brain of Marsupials j 3, the brain of 

 Semi-apes; 4, the brain of Apes; 5, the brain of Mav^Ufca ApM| 4 the 

 brain of Ape-men j and 7| the brain of Man. 



