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 mh), 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 

 {mesoderraa), 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 (lis) and the skin-fibrous layer Qif), 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 bilateial (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 

 waUs 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. QQ mf) and finally coalesce, so that the trench 

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

 Along the longitudinal axis of the body, a solid cylindriail 



