The cells forming the roof of the cavity pass without any sharp boundary into the yolk-cells, there being at the junction of the two a number of cells of an intermediate character. The cells both of the roof and the floor continue to increase in number, and those of the roof become divided into two distinct strata (fig. 70, _ep_). The upper of these is formed of a single row of somewhat cubical cells, and the lower of several rows of more rounded cells. Both of these strata eventually become the epiblast, of which they form the epidermic and nervous layers. The roof of the segmentation cavity appears therefore to be entirely constituted of epiblast. The next changes which take place lead (1) to the formation of the mesenteron[48], and (2) to the enclosure of the yolk-cells by the epiblast. [48] Since the body cavity is not developed as diverticula from the cavity of invagination, the latter cavity may conveniently be called the mesenteron and not the archenteron. The mesenteron is formed as in Petromyzon and Lepidosteus by an unsymmetrical form of invagination. The invagination first commences by an inflection of the epiblast-cells for a small arc on the equatorial line which marks the junction between the epiblastic cells and the yolk-cells (fig. 70, _x_). The inflected cells become continuous with the adjoining cells; and the region where the inflection is formed constitutes a kind of lip, below which a slit-like cavity is soon established. This lip is equivalent to the embryonic rim of the Elasmobranch blastoderm, and the cavity beneath it is the rudiment of the mesenteron. [FIG. 70. SECTION THROUGH FROG'S OVUM AT THE CLOSE OF SEGMENTATION. (After Götte.) _sg._ segmentation cavity; _ll._ large yolk-containing cells; _ep._ small cells at formative pole (epiblast); _x._ point of inflection of epiblast; _y._ small cells close to junction of the epiblast and yolk.]