THE '^ TREAD, OR CICATRICULA. 



283 



from the large yelk-ball which lies beneath it. The fiat 

 layers become tubes, in consequence of their edges inclining 



Fig. 70. — Separation of the disc-shaped mammalian germ from the 

 yelk-sac; in transverse section (diagrammatic). A. The germ-disc ih,hf) 

 lies flat on one side of the intestinal germ- vesicle (kb). B. The spinal tube 

 (mr) appears in the centre of the germ-disc, and underneath fhis the 

 notochord (ch). C. The intestinal-fibrous layer (df) has groven round 

 the intestinal-glandular layer {del). D. The skin-fibrous layer (hf) and the 

 iutestinal-fibrous layer {df) separate in the outer wall; the intestine (cJ) 

 begins to separate itself from the navel- vesicle {nh). E. The intestinal tube 

 (nir) is closed ; the body -cavity (c) begins to form. F. The primitive verte- 

 brae (w) separate ; the intestine {d) is almost entirely closed. G. The 

 primitive vertebrae {iv) begin to grow round the spinal tube (mr) and the 

 notochord {ch) ; the intestine (d) is separate from the navel-vesicle (nb). 

 H. The vertebrae (u) have grown round the spinal tube (mr) and the noto- 

 chord ; the body -cavity (c) is closed, the navel-vesicle has disappeared. The 

 amnion and serous membrane are omitted. 



In all, the letters indicate the same parts : h, horn-plate ; mr, spinal 

 tube ; hf, skin-fibrous layer ; w, primitive vertebrae ; ch, notochord ; c, body- 

 cavity (cceloma) ; df, intestinal-fibrous layer ; dd, intestinal-glandular layer; 

 d, intestinal cavity ; 7iJ), navel-vesicle. 



