3O8 THE EVOLUTION OF MAN. 



period, into two distinct cavities by the formation of the 

 diaphragm ; these are the chest, or thoracic cavity, and the 

 abdominal cavity. Immediately below the mesentery -plate, 

 in the gap between the intestinal-glandular layer, the in- 

 testinal-fibrous layer, and the primitive vertebral bands, 

 another organ appears at an early stage, in the form of a 

 tube with a thin wall (Fig. 92, ao). This is the first rudi- 

 ment of a large blood-vessel, the primitive artery, or aorta. 

 It arises by fission from the intestinal-fibrous layer. 



During these processes the inner germ-layer, the intes- 

 tinal-glandular layer (Fig. 92, df), remains quite unaltered, 

 and it is only somewhat later that it begins to show a very 

 uhallow, channel-like depression along the central line of the 

 germ-shield, immediately below the notochord. This is the 

 intestinal channel, or intestinal furrow, and it already indi- 

 cates the future destination of this germ-layer. For as the 

 intestinal channel gradually deepens, and its lower edges 

 bend towards one another, it assumes the form of a closed 

 tube, the intestinal tube, precisely as the dorsal furrow 

 became the spinal or medullary tube (Fig. 92). The in- 

 testinal-fibrous layer (/), which lies on the intestinal-glan- 

 dular layer (d), naturally follows the curve of the latter. 

 Thus f -om the time when it first begins to develop, the 

 intestinal wall is composed of two strata, internally oi 

 the intestinal-glandular layer, externally of the intestinal- 

 fibrous layer. 



The formation of the intestinal tube~ is so far similar to 

 that of the spinal tube, that in both cases a rectilineal trench, 

 or furrow, first appears along the central line of a flat germ- 

 layer. The edges of this furrow then incline towards each 

 other, and by coalescence form a, tube (Fig. 93). But the 



