The Development of the Frog 27 



10). The mouth and true anus will not be 

 formed until later. 



The noiochord, or primitive backbone, is 

 formed at this time as a rod-like thickening of 



Fig. 10. — A transverse section through thk mid- 

 dle OF the length of a frog embryo, at about the 

 STAGE REPRESENTED IN FiG. g. (After Marshall.) 



C//, notochord. -£", ectoblast. i/jT/, pouch-like diverticulum 

 of the entoblast into the mesoblast. M, mesoblast. N G, neural 

 groove. N P^ neural plate, 7", mesenteron. K, yolk. 



the entoblast, extending along the roof of the 

 mesenteron in the middle line for the greater 

 part of its length. It does not, for a time, sepa- 

 rate from the rest of the entoblast and is very 

 indistinct ; but it later forms a distinct rod of 



