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 



7 



Fir,. 70. Separation of the disc-shaped mammalian germ from the 

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

 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 this 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 (nl). E. The intestinal tube 

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

 bras (w) separate ; the intestine (d) is almost entirely closed. 0. Tho 

 primitive vertebrae (w) begin to grow round the spinal tube (mr) and tho 

 notochord (c/i) ; the intestine (d) is separate from the navel-vesicle (nb). 

 H. The vertebrao (w) 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 : li, horn-plate ; mr, spinal 

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

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

 d, intestinal cavity ; ?ib, navel-vesicle. 



