ORIGIN OF THE FUNCTIONS OF THE BRAIN. 229 



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

 (rneninx arachnoides), and the outer membrane (dura 

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

 layer. 



TABLE XXVII. 



Systematic Survey of the most important Periods in the Phylogeny 



,of the Human Skin-coverings. 



I. First Period : Skin of Gastrceads. 



The 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 gastrula of the 

 Amphioxus. 



II. Second Period : Skin, of Primitive Worms. 



The simple exoderm of the Gastrasad 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 : Skin 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 and skeleton plates. 



