OBIQIN OF THE FUNCTIONS OF THE BRAIN. 229 



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

 (meniTix arachnoides), and the outer membrane {dura 

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

 layer. 



TABLE XXVII. 



Ststkmatic Susvet of the most important Periods ih th« PHTLOonrr 



OP THE Human Skin-coverings. 



I. First Period : Skin of Gastrceads. 



The entire skin-oovering (including the nervons 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 

 Amphiozns. 



n. Second Period : Shin of Primitive Worma, 



The simple 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 nund. 



m. Third Period : Skin of Chordonia. 



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

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

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

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

 in all Worms). 



rV. Fowrth Period : Skin of Acranick. 



The horn-plate yet forms a simple epidermis. The leather-plate ii fUly 

 differentiated from the mnsole and skeleton plates. 



