I2 4 



FORMATION OF THE LAYERS. 



mesoblast extends further inwards, this slit enlarges, especially 

 at its inner end, into a considerable cavity ; the blind end of 

 which is separated by a narrow layer of yolk-cells from the 

 segmentation cavity (fig. 71 B). 



In the course of the involution, the segmentation cavity 

 becomes gradually pushed to one side and finally obliterated. 

 Before obliteration, it appears in some forms (Pelobates fuscus) to 

 become completely enclosed in the yolk-cells. 



While the invagination to form the mesenteron takes place 

 as above described, the enclosure of the yolk has been rapidly 

 proceeding. It is effected by the epiblast growing over the yolk 

 at all points of its circumference. The nature of the growth is 

 however very different at the embryonic rim and elsewhere. At 

 the embryonic rim it takes place by the simple growth of the 

 rim, so that the point x in figs. 70 and 71 is carried further and 

 A B 



FIG. 71. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF 

 A FROG AT TWO STAGES, TO SHEW THE FORMATION OF THE GERMINAL LAYERS. 

 (Modified from Gotte.) 



ep. epiblast; m, dorsal mesoblast; m'. ventral mesoblast; hy. hypoblast; 

 yk. yolk ; jr. point of junction of the epiblast and hypoblast at the dorsal side of the 

 blastopore ; al. mesenteron ; sg. segmentation cavity. 



further over the surface of the yolk. Elsewhere the epiblast at 

 first extends over the yolk as in a typical epibolic gastrula, with- 

 out being inflected to form a definite lip. While a considerable 

 patch of yolk is still left uncovered, the whole of the edge of the 

 epiblast becomes however inflected, as at the embryonic rim 

 (fig. 71 A); and a circular blastopore is established, round the 



