230 THE EVOLUTION OF MAN. 



V. Fifth Period : STcin of Cyclostoma. 



The outer-skin remains a simple, soft mncons layer of cells, but forma 

 one-celled glands (cup-cells). The leather-skin (corium) differentiates into 

 cutis and aub-cutis. 



VI. Sixth Period : Skin of Primitive Fishes. 



The outer skin is still simple. The leather skin forms placoid scales or 

 small bony tablets, as in the Selachii. 



VII. Seventh Period : Skin of Amphibia. 



The outer skin differentiates into an outer horn-layer, and au inner 

 mucoua layer. The ends of the toes are covered with horny sheaths (first 

 rudiments of claws or nails). 



VIII. Eiyhth Period : Skin of Mammals. 



The outer skin forms the appendages characteristic of Mammals only j 

 hair, and sebaceous, sweat, and milk glands. 



TABLE XXVIII. 



SYSTEMATIC SURVEY OF THE MOST IMPORTANT PERIODS IN THE PHYLOGEKT 

 OF THE HUMAN NERVOUS SYSTEM. 



I. First Period : Medulla of Gastrceads. 



Tho nerve system is not yet distinct from the skin, and, together with 

 the latter, is represented by the simple cell-stratnm of the exodenr., or 

 primary skin-layer; as it is at the present day in the gastrula of the 

 Amphioxus. 



II. Second Period : Medulla of Primitive Worms. 



The central nerve system is yet, at first, a part of the skin-sensory layer, 

 and afterwards consists of a throat medulla, a simple nerve-ganglion lying 

 above the throat ; as it is now in the lower Worms : the upper throat 

 iganglion. 



