UlSTOUY OF ZOOLOGY. 



39 



yyip 







I' IG. n.^Cross-spctinns thrniifrli the dorsal reRion of Tritnn emhryos iit diflforent -icps 

 lirom O. Hertwig). In ftlir; meduUarv plate (anlage of spinal cord) »i/> is marKca 

 "ff from the skin fcpidcrmis, <•/)) h\- the medullary folds (mf). In II the i"*"""': 

 hiry plate hv inrollintr of the medullarv folds is converted into a groove, in "' 

 tlie groove has cdosed into a tuhe (ii). the spinal cord, tvhieh has separated from 

 the rest of the ectoderm (epidermisl. r, hody cavity (codom) ; rh, notochord ; 

 <'V, cavity of primitive somite (myotomel; d0, ydlk-cells; ik, entoderm; lij, lumen 

 of gut; 7)ifc', mfc^ somatic and splanchnic layers of mesoderm. 



