The Development of the Frog 27 



10). The mouth and true anus will not be 

 formed until later. 



The notochord, or primitive backbone, is 

 formed at this time as a rod-like thickening of 



CH 



HM 



FlG. 10. A TRANSVERSE SECTION THROUGH THE MID- 

 DLE OF THE LENGTH OF A FROG EMBRYO, AT ABOUT THE 



STAGE REPRESENTED IN FIG. 9. (After Marshall.) 



C//, notochord. E, ectoblast. HM, pouch-like diverticulum 

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

 groove. N P, neural plate. 7 1 , 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 



