Now a second, narrower but longer, cavity arises by a process of folding at the lower pole, and by the falling away from each other of the white entoderm-cells (Figs. 41–44 _N_). This is the primitive gut-cavity or the gastric cavity of the gastrula, progaster or archenteron. It was first observed in the ovum of the amphibia by Rusconi, and so called the Rusconian cavity. The reason of its peculiar narrowness here is that it is, for the most part, full of yelk-cells of the entoderm. These also stop up the whole of the wide opening of the primitive mouth, and form what is known as the “yelk-stopper,” which is seen freely at the white round spot at the south pole (_P_). Around it the ectoderm is much thicker, and forms the border of the primitive mouth, the most important part of the embryo (Fig. 44 _k, k′_). Soon the primitive gut-cavity stretches further and further at the expense of the segmentation-cavity (_F_), until at last the latter disappears altogether. The two cavities are only separated by a thin partition (Fig. 43 _s_). With the formation of the primitive gut our frog-embryo has reached the gastrula stage, though it is clear that this cenogenetic amphibian gastrula is very different from the real palingenetic gastrula we have considered (Figs. 30–36). In the growth of this hooded gastrula we cannot sharply mark off the various stages which we distinguish successively in the bell-gastrula as morula and gastrula. Nevertheless, it is not difficult to reduce the whole cenogenetic or disturbed development of this amphigastrula to the true palingenetic formation of the archigastrula of the amphioxus. Fig.46. Embryonic vesicle of triton. Fig. 46—Embryonic vesicle of triton (_blastula_), outer view, with the transverse fold of the primitive mouth (_u_). (From _Hertwig._)