HISTORY OF ZOOLOGY. 



39 



FIG. ^.-Cross-sections through the dorsal region of Triton embryos at .different ages 

 (from O. Hertwig) In T the medullary plate fanlage of spinal cord) rn is 

 off from the skin (epidermis, ep} bv the medullary folds (wf). In II the 

 lary plate by inrolling of the medullary folds is converted into a groove 



In 11 



the groove has closed into a tube <n\ the spinal cord, which has separated trom 

 the rest of the ectoderm (epidermis). C, body cavity (coelom) ; cfc, notochord ; 

 cp, cavity of primitive somite (myotome); rfz, yolk-cells; i/c, entoderm; Ig, lumen 

 of gut ; wfc 1 , rnfe*, somatic and splanchnic layers of mesoderm. 



