114 



DEVELOPMENT OF THE FROG 



followed, the black epiblast cells seeming to spread over and 

 gradually almost enclose the white yolk-cells. 



This apparent spreading of the epiblast does not take 

 place equally fast all round its margin, and at one place the 

 epiblast, instead of extending over the yolk- cells, bends 

 inwards towards the interior of the egg. This place is 

 visible externally as a sharply defined horizontal or slightly 

 crescentic groove, concave downwards, bounded above by the 

 small black epiblast cells, and below by the -large white yolk- 



FIG. 25. Longitudinal vertical section through a frog embryo show- 

 ing the completion of the mesenteron. 



B, blastopore ; EE, epidermic layer of epiblast ; EN, nervous layer 

 of epiblast ; H, invaginate hypoblast ; M, mesoblast ; MM, mesenteron ; 

 N. notochord. 



cells. As the epiblast continues spreading over the rest of 

 the yolk this groove becomes horseshoe-shaped, and a little 

 later circular. 



If we examine the egg over a mirror, it will be seen that 

 this groove does not remain stationary but shifts gradually 

 from near the equator (DL, Fig. 22 A) towards the lower 

 pole, and when it has reached this point it has assumed 

 a circular form (Fig. 24A). In other words the dorsal, 



