THE "TREAD," OR CICATRICULA. 



28 3 



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 

 (mr) 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 

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

 brae (w) separate ; the intestine (d) is almost ent'rely 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 vertebrae (ir) have grown round the spinal tube (mr) and the noto- 

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

 amnion and serous membrane are omitted. 



In all, the letters indicate the same parts : li, horn-plate ; mr, spinel 

 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 ; nb, navel-vesicle. 

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