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 XXYII. 



Systematic Survey of the most important Periods in the Phylogeny 



OF the Human Skin-coverings. 



I. First Period : Sldn of Gastrocads. 



The entire skm-coveritij^ (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 TTorms. 



The sim^ile exoderm of the Gastraead 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 {eindermis) , 

 and the central marrow (upi^er 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 hom-plate yet forms a simple epidermis. The leather-plate is fully 

 differentiated from the muscle and skeleton plates. 



