at the south pole, where the large and clear cells of the lower half are visible. This white region at the south pole corresponds, as we shall see afterwards, to the primitive mouth of the gastrula. The whole mass of the inner and larger and clearer cells (including the white polar region) belongs to the entoderm or ventral layer. The outer envelope of dark smaller cells forms the ectoderm or skin-layer.
Figs. 41-44. Four vertical sections of the fertilised ovum of the toad, in four successive stages of development. Figs. 41–44—Four vertical sections of the fertilised ovum of the toad, in four successive stages of development. The letters have the same meaning throughout: _F_ segmentation-cavity. _D_ covering of same (_D_ dorsal half of the embryo, _P_ ventral half). _P_ yelk-stopper (white round field at the lower pole). _Z_ yelk-cells of the entoderm (Remak’s “glandular embryo”). _N_ primitive gut cavity (progaster or Rusconian alimentary cavity). The primitive mouth (prostoma) is closed by the yelk-stopper, _P. s_ partition between the primitive gut cavity (_N_) and the segmentation cavity (_F_). _k k′,_ section of the large circular lip-border of the primitive mouth (the Rusconian anus). The line of dots between _k_ and _k′_ indicates the earlier connection of the yelk-stopper (_P_) with the central mass of the yelk-cells (_Z_). In Fig. 44 the ovum has turned 90°, so that the back of the embryo is uppermost and the ventral side down. (From _Stricker._).
Blastula of the water-salamander. Fig. 45—Blastula of the water-salamander (_Triton_). _fh_ segmentation-cavity, _dz_ yelk-cells, _rz_ border-zone. (From _Hertwig._)
In the meantime, a large cavity, full of fluid, has been formed within the globular body—the segmentation-cavity or embryonic cavity (_blastocœl,_ Figs. 41–44 _F_). It extends considerably as the cleavage proceeds, and afterwards assumes an almost semi-circular form (Fig. 41 _F_). The frog-embryo now represents a modified embryonic vesicle or _blastula,_ with hollow animal half and solid vegetal half.
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).