FORMATION OF THE INTESTINAL TUBE. 



309 



two processes are in reality quite different. For the spinal 

 tube closes along throughout its entire length into a cylin- 



Fig. 93. — Three diagrammatic transverse sections through the germ- 

 shield of a higher Vertebrate, showing the origin of the tubular rudimentary 

 organs from the bent germ-layers. In Fig. A the spinal tube (n) and the 

 intestinal tube (a) are still open trenches ; the primitive kidneys (w) are still 

 simple skin-glands. In Fig. B the spinal tube (n) and the dorsal wall have 

 already closed, while the intestinal tube (a) and the ventral wall are still 

 open ; the primitive kidneys are pinched off. In Fig. C both the spinal tube 

 with the dorsal wall above, and the intestinal tube with the ventral wall 

 below, are closed. All the open trenches have become closed tubes ; the 

 primitive kidneys have penetrated into the interior. In all three figures the 

 letters indicate the same parts : h, skin-sensory layer ; n, spinal tube, or 

 medullary tube ; u, primitive kidneys ; as, notochord ; s, vertebral rudiments ; 

 r, dorsal wall; b, ventral wall ; c, body-cavity (cceloma^) ; f, intestinal-fibrous 

 layer: t, primitive artery (aorta); v, primitive vein (intestinal vein); d, 

 intestinal-glandular layer ; a, intestiual tube. (Cf. Plates IV. and V.) 



drical tube, while the intestinal tube remains open in the 

 middle, and, till a much later stage, this cavity x'emains 

 connected with the cavity of the intestinal germ-vesicle. 

 The connection between these two cavities is closed only at 

 a very late period, by the formation of the navel. The 

 closing of the medullary tube proceeds from both sides, the 

 right and left edges of the dorsal furrow coalescing. The 



