230 THE EVOLUTION OF MAN. 



y. Fifth Period : SMn of Cyclostoma. 



The onter-skin remains a simple, soft mucous layer of cells, but forma 

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

 cutis and suh-cuiis. 



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 : Slcin of Amphibia, 



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

 mucous layer. The ends of the toes are covered with homy sheaths (first 

 rudiments of claws or nails). 



VIII. Fvjhth Period : Shin of Mammals. 



The outer skin forms the appendages characteristic of Mammals only ; 

 hair, and sebaceous, sweat, and milk glands. 



TABLE XXYIII. 



Systematic Sukvey of the most important Periods in the Phylogent 



or THE Human Nervous System. 



I. First Period : Medulla of Gastrceads, 



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

 the latter, is represented by the simple cell-stratum of the exoderm, or 

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

 Anaphioxua. 



II. Second Period : Medulla of Primitive Worms. 



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

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

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

 jganglion. 



