278 THE EVOLUTION OF MAN. 



grammatic transverse sections through those germ-forms 

 which first develop from the Gastrula, will best and most 

 easily afford us the desired view. (Cf. Fig. 62-69, and 

 Plates IV., V.) In the first place, a third layer, the middle 

 layer, or fibrous layer (mesoderma, Fig. 63 mb), arises be- 

 tween the two primary germ-layers of the Gastrula (Fig. 

 62). Then, this three-layered stage is followed by one in 

 which there are four layers (Fig. 64). As we have already 

 stated, each of the two primary germ-layers probably 

 originally contributed to the formation of the middle layer 

 (mesoderma), although it is usually asserted that the latter 

 originates from one only of the former. It is probable that 

 the exoderm, or skin-layer (e), separated into the skin- 

 sensory layer (ha) and the skin-fibrous layer (hf), and 

 correspondingly, the entoderm, or intestinal layer, into the 

 intestinal-fibrous layer (df) and the intestinal-glandular 

 layer (dd). 



When the four germ-layers are completed, the form of 

 the Gastrula, which had but one axis, has become symme- 

 trically bilateral (cf. p. 257). In consequence of the body 

 becoming flat, a distinction is formed between the dorsal 

 and ventral sides, between the right and the left. Parallel 

 with the axis of length, a delicate streak, the indication of 

 a furrow, appears in the centre of the dorsal side. The side 

 walls of this furrow, which is called the " spinal furrow " 

 (m/), rise in the form of two parallel ledges (Fig. 65 m/) ; 

 these are the spinal swellings (medullary or dorsal swell- 

 ings). Their two parallel edges bend toward each other 

 (Fig. 66 mf) and finally coalesce, so that the trench 

 becomes a tube ; this is the spinal tube (Fig. 67 rar). 

 Along the longitudinal axis of the body, a solid cylindrical 



