THE "TREAD," OR CICATRICULA. 



283 



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

 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 {h,hf) 

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

 (rar) appears in the centre of the germ-disc, and underneath t'his the 

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

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

 intestinal-fibrous layer (df) separate in the outer wall ; the intestine (d ) 

 begins to separate itself from the navel- vesicle (nb). E. The intestinal tube 

 (rar) 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 (w) begin to grow round the spinal tube (mr) and the 

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

 H. The vertebras (w) have grown round the spinal tube (rar) and the noto- 

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

 amnion and serous membrane are omitted. 



In all, t e le.ters 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-hbrous layer; dd, intestinal-glandular layer j 

 d, intestinal cavity ; nb, navel-vesicle. 

 21 



