HISTORY OF ZOOLOGY 



mf mf 



31 



mf 



III. 



Fig. 9. — Cross-sections through the dorsal region of Tritmr embryos at different ages 

 (from O. Hertwig). In I the medullary plate (anlage of spinal cord) mp is marl^ced 

 off from the skin (epidermis, ep) by the medullary folds {mf). In // the medullary 

 plate, by inrolUng of the medullary folds, is converted into a groove. In /// the groove 

 has closed into a tube («), the spinal cord, which has separated from the rest of the 

 ectoderm fepidermis). r, body cavity (coelom); ch, notochord; (*/>, cavity of primitive 

 somite (myotome); dz, j-olk-cells; ik, entoderm; Ig, lumen of gut; mk^, ink'-, somatic 

 and splanchnic layers of mesoderm; n, spinal cord. 



