ORIGIN OF THE FUNCTIONS OF THE BRAIN. 229 



are the inner membrane (pia mater), the central membrane 

 (meninx arachnoides), and the outer membrane {dura 

 'mater). All these parts are developed from the skin-fibrous 

 layer. 



TABLE XXVII. 



Ststematic Suevey or the most impoktant Peetods in the Phklogeny 

 OF the Human Skin-coyekings. 



I. First Period : Skin of Gastrceads. 



Tlie entire skin-covering (including the nervous system, not yet differ- 

 entiated from it) consists of one simple layer of ciliated cells (exoderm, or 

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

 Amphioxus. 



II. Secoitd Period : Skin. 0/ Primitive Worms. 



The simple exoderm of the Gastrjead has thickened and split into two 

 distinct layers, or secondary germ-layers : the skin-sensory layer (rudiment 

 of the horn-plate and nerve-system) and the skin-fibrous layer (rudiment of 

 the leather skin (corium), the muscle-plate and the skeleton-plate. The 

 skin is potentially both covering and mind. 



III. Third Period : SMn of Chordoma. 



The skin-sensory layer has differentiated into the horn-plate (epidermis) , 

 and the central marrow (upper throat ganglia) separated from it ; the latter 

 elongates into a medullary tube. The skin-fibrous layer has differentiated 

 into the leather plate (corium) and, below this, the skin-muscular pouch (as 

 in all Worms). 



IV. Fourth Period ; Skin of Acrania. 



The horn-plate yet forms a simple epidermis. The leather-plate is fully 

 differentiated from the muscle ajid skeleton plates. 



