THE MESENTERON. 



107 



circular aperture in the epiblast, defined by this groove, is spoken 

 of as the blastopore, or anus of Rusconi ; and the mass of yolk- 

 cells which fills up the aperture, as the yolk-plug. 



The blastopore lies at first at the lower pole of the egg. 

 Reference to Figs. 54, 55, and 58 will show that this lower pole 

 becomes subsequently the hinder or tail end of the embryo, so 

 that the lips of the blastopore, BP and BP', may be spoken of 

 as dorsal and ventral respectively. 



From the figures, and from the above description, it will be 



FIG. 53. Horizontal section across a Frog Embryo of the same age as that 

 shown in Fig. 52, the section being taken along a line joining the refer- 

 ence letters T and B in Fig. 52. x 25. 



B, blastoccel or segmentation cavity. EE, outer or epidermic layer of epiblast. 

 EN, inner or nervous layer of epiblast. TT, liypoblast. M, mesoblast. T, mesenteroii. 

 Y, yolk-cells. 



seen that the groove which limits the blastopore appears first 

 at its dorsal margin, BP, and spreads round the sides to the 

 ventral margin, BP'. The slit extends at first radially inwards, 

 towards the centre of the egg (Figs. 51, BP, and 52, BP') ; but 

 ajong the dorsal surface the slit, after a short radial course 

 (Fig. 52, BP), turns sharply at right angles (Fig. 52, T), and 

 spreads forwards concentrically with the surface of the embryo. 



The whole embryo, which up to this stage has been spherical, 

 now begins to elongate, becoming ellipsoidal, with the blastopore 



