230 THE EVOLUTION OF MAJNT. 



V. Fifth Period : Skin of Cyelostoma. 



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

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

 cutis and suh-cutis. 



VI. Sixth Period : Skin of Prtmitive Fishes. 



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

 small bony tablets, as in the Selachii. 



VII. Seventh Period : Skin of Amphibia, 



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

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

 rudiments of^lslaws or nails). 



VIII. Eighth Period : Skin of Mammals. 



The outer skin forms the appendages characteristic of Mammals cmly ; 

 hair, and sebaceous, sweat, and milk glands. 



TABLE XXVIII. 



Systematic Suevei of the most important Periods in the PHTLOoiirr 



OF THE Human Nervous System. 



I. First Period ; Medulla of QastroBods. 



The nOTve 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 

 AmphiozQS. 



n. 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- 

 ganglion. 



