THE MESENTEEON. 



107 



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

 of as tl^ e blastopo re, or anus of Rusconi ; and the mass of yolk- 

 cells which fills up the aperture, as the Yolk-pltt g. 



Th e blastopore lies at fi rst at the lower pole of the egg. 

 Reference to li'igs. 54, 55, and b8 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 Embrj'o 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. 62. x 25. 



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

 EN, inner or nervous layer of epiblast. H, liypoblast. M, mesoblast. 'T, mesenteron. 

 Y, yolk-cells. 



seen that the groo ve which l imits the blastopore appears first 

 at its d orsal margin, bp, a nd~spreads round the si^tes to the 

 ventral mar gin, bT . The slit extends at first radially inwards, 

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

 along 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 



