122 



DEVELOPMENT OF THE FROG. 



small circular patch where alone the white yolk-cells come to 

 the surface, Fig. 25, YP. This extension of the epiblast occurs 

 all round its margin ; it is effected by the addition of cells cut 

 out from the superficial layer of yolk-cells, and not by the 

 epiblast cells overlapping or spreading over the yolk-cells. 



Fig. 26. Longitudinal vertical section through a frog embryo showing 

 the completion of the mesenteron. 



B, blastopore : E E, epidermic layer of epiblast : E N, nervous layer of 

 epiblast : H, invaginate hypoblast : M. mesoblast : M N, mesenteron : 

 N, notochord. 



The circular aperture in the epiblast where the yolk-cells still 

 come to the surface is called the blastopore : Fig. 26, B. It is 

 situated at what will become the posterior end of the embryo ; 

 and it is bordered by a distinct rim or lip, round which the 

 epiblast turns inwards into the interior of the egg. The circular 

 plug of yolk cells filling up the blastopore is spoken of as the 

 yolk plug. 



The Mesenteron. During this time a narrow slit-like cavity 

 has appeared in the interior of the egg : Fig. 25, MN. This 

 cavity, the mesenteron, is formed by splitting apart of the 

 yolk cells, and from its first appearance communicates with 

 the exterior at the blastopore. It is at first short and ver}' 

 shallow. Figs. 24 and 25 ; but it rapidly extends further and 

 further into the egg, and by depression of its lower w^all or 

 floor becomes widened out into a space of considerable size. 



