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.