112 



DEVELOPMENT OF THE FROG 



more spherical, less strongly pigmented, and loosely arranged 

 in a layer two or more cells deep. 



The epiblast cells continue to increase by division, and very 

 early, owing apparently to multiplication of the cells at the 

 margin of the layer, seem to spread over the lower or yolk-cells. 

 Owing to the difference in colour of the two halves of the egg, 

 the various stages of this process can be readily followed, the 

 black epiblast cells seeming to spread over and gradually enclose 

 the almost white yolk-ceUs, until a small round patch alone 



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 ; MN, mesenteron ; 

 N, notochord. 



remains uncovered. It is doubtful how far this process is due 

 to actual growth of the epiblast, rather than to differentiation 

 of lower layer 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 into the 

 latter, and grows into 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- 

 cells. As the epiblast continues spreading over the rest of the 



