304 THE EVOLUTION OF MAN. 



drawn : thus, for instance, the two middle, secondary germ- 

 layers are usually called the skin-fibre plate and the intes- 

 tinal-fibre plate (instead of layers, or leaves). Conversely, 

 the horn-plate (which is a portion of the skin-sensory 

 layer) is usually called the horn-layer, or leaf. As far as 

 possible we shall, however, maintain this important distinc- 

 tion : we shall only use the term layers, or leaves, of the 

 two primary, and the four secondary germ-layers ; naturally, 

 however, we must speak of the side-plates as side-layers, or 

 leaves, as they first originate by a coalescence of the two 

 primary germ-layers. On the other hand, we shall speak 

 of the so-called horn-layer and of all the layer-like rudi- 

 mentary organs, which are split off or differentiated from 

 the four layers, or leaves, as plates; e.g. the muscle-plate, 

 etc. 



After the chorda has entirely separated from the two 

 side-layers, a portion, in the shape of a long, thick cord, 

 breaks off, in the posterior portion of the germ-shield, from 

 ,the inner edge of each of the side-layers (Fig. 90, uwp, 91, u}. 

 We will call this the primitive vertebral plate, or better, 

 the primitive vertebral cord, for it afterwards develops into 



FIG. 91. Transverse section through the germ-shield of a Chick (at the 

 end of the first day), rather more developed than in Fig. 90 ; about twenty 

 times the natural size. The two edges of the spinal plate (m), which, as 

 spinal swellings (iv), separate the latter plate from the horn-plate (li), incline 

 towards each other. On both sides of the notochord (c7i) the inner portion 

 of the side-layers (u) has separated itself as a primitive vertebral band 

 from the outer portion. The intestinal-glandular layer (d) is not yet 

 modified. (After Eemak.) 



