306 THE EVOLUTION OF MAN. 



the Theory of Descent explains it as but a perfectly natural 

 process. It is quite natural that the central nervous system, 

 the organ by which all intercourse with the outer world, 

 all mental activities, and all sensory perception are accom- 

 plished, should be developed by detachment from the outer 

 skin {epidermis). At a later stage the medullary tube 

 separates entirely from the outer germ-layer, is surrounded 

 by the primitive vertebrae, and is forced inwards. From this 

 time, the remaining portion of the skin-sensory layer (Fig. 

 92 h), is called the horn-plate or " horn-layer," because the 





c/i uw ao *p ^ j yy 



Fig. 92. — Transverse section through the germ-shield of a Chick (second 

 day of incubation) ; about 100 times the natural size. In the outer germ- 

 layer, the axial dorsal furrow, having completely closed, forms the spinal 

 tube (mr), and has pinched itself off from the horn-plate (h). In the middle 

 germ-layer, the axial notochord (cTi) is entirely separated f rom the primitive 

 vertebral bands (uw), in the interior of which a transitory cavity (uwh) 

 afterwards forms. The side-layers have split into the outer skin-fibrous- 

 layer (hpl) and the inner intestinal -fibrous layer (df), the two being still 

 connected by the middle plates (mp). The fissure (sp) between the two is 

 the first rudiment of the body-cavity (cceloma). In the gap between the 

 primitive vertebral bands and the side-layers, on either side, is the primitive 

 kidney (ung), and on the inside the primitive artery (ao). (After Kolliker.) 



outer skin (epidermis), with its horny appendages — nails, 

 hair, etc. — develops from it. (Cf. Plates IV. and V.) 



At a very early period, in addition to the central nervous 

 system another or wholly different organ is seen to arise 

 from the outer skin ; this is the primitive kidney, which 



