230 THE EVOLUTION OP MAN. 



v. Fifth Period ; Shin of Cyclostoma. 



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

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

 cutis and suh-cuiis. 



VI. Sixth Period : Slcin of Primitive Fishes. 



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

 small bony tablets, as in the Selaohii. 



VII. Seventh Period : STcin of Amphibia. 



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

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

 rudiments of claws or nails). 



VIII. Eighth Period : Skin of Mammals. 



The outer skin forms the appendages characteristic of Mammals only ; 

 hair, and sebaceous, sweat, and milk glands. 



TABLE XXVIII. 



Systematic Sukvev of the most important Pekiods in the Phyiogent 

 or THE Human Neeyous 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 skiri-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, apart 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- 

 ganglion. 



