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 (to), separate the latter plate from the horn-plate (h), incline 

 towards each other. On both sides of the notochord (cTi) 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 Remak.) 



