508 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 

 shallow, 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 th<° 



