300 



THE EVOLUTION OF MAN. 



studying these sections, one by one, in every stage of the 

 evolution, that it is possible fully to understand the pro- 

 cesses by which the exceedingly complex structure of the 

 vertebrate body is developed from the simple leaf-shaped 

 germ-shield. 



If we now make a perpendicular section through the 

 sole-shaped germ-shield (Fig. 86, 87), the first thing we 

 notice is the difference between the three germ-layers, as 

 they lie one over the other (Fig. 88). The germ-shield con- 

 sists, as it were, of three shoe-soles overlying each other. 

 The undermost, or innermost, of these (the intestinal -gland- 

 ular layer) is the thinnest stratum, and consists of a single 

 layer of cells (Fig. 88 d). The middle of these shoe-shaped 

 bodies (the mesoderm) is considerably thicker and more or 

 less evidently appears to be composed of two closely con- 

 nected layers. The third and uppermost, or outermost sole- 



Fro. 88. Transverse section through the germ-disc of a Chick (a few 

 hours after the beginning of incubation) : h, skin-sensory layer; m, skin- 

 fibrous layer ; /, intestinal-fibrous layer (the two latter are united into the 

 middle-layer, or mesoderm) ; d, intestinal-glandular layer. All the four 

 secondary germ-layers have coalesced in the middle and from the thick 

 axis-band (xy~) n, first trace of the primitive groove ; it, region of the 

 future primitive kidney rudiment. (After Waldeyer.) 



shaped body (A), is the skin-sensory layer, and consists of 

 smaller and lighter-coloured cells. In the middle of the 

 transverse section, along a considerable part of the longi- 



