SURVEY OF HUMAN NERVOUS SYSTEM. 23 1 



III. Third Period : Medulla of Chordonia. 



The central nerve systena consists of a simple medullary tube, an 

 elongation of the upper throat ganglion, which is separated from the intes- 

 tine by a notochord {chorda dorsalis). 



IV. Fourth Period : Medulla of Acrania. 



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

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

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

 extremity of the long cylindrical spinal marrow. 



V. Fifth Period : Medulla of Cyclostoma. 



The simple, bladder-like radiment of the brain divides into five con- 

 secutive brain-bladders of simple structure. 



YI. Sixth Period : Medulla of Primitive Fishes. 



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

 permanently retained by the Selachii. 



VII. Seventh Period : Medulla of Amphibia, 



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

 which is now characteristic of the brain in Amphibia. 



YIII. Eighth Period : Medulla of Mamvials. 



The brain attains the characteristic peculiarities distinctive of Mammals. 

 The following may be diGtinguished as subordinate stages of development j 

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

 Semi-apes ; 4, the brain of Apes ; 5, the brain of Man-like Apes j 6, the 

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



