In the section immediately behind section fig. 11 the mesoblast was thickened, but had not become split. In fig. 10 the hypoblast folds are seen to have united in the centre, so as to form a completely closed digestive canal (_d_); the folds of the mesoblast have also united, so that there is only a single cavity in the heart (_hz_), lined, as was the case with the omphalomeseraic veins, by endothelium. In conclusion, I have to thank Dr Foster for his assistance and suggestions throughout the investigations which have formed the subject of these three short papers, and which were well carried on in the apartments used by him as a Physiological Laboratory. EXPLANATION OF PLATE 2. Fig. 1 is taken from the anterior part of the pellucid area of a thirty hours' chick, with four protovertebræ. At _n_ is a nucleus with two nucleoli. Figs. 2 and 3 are taken from the posterior end of the pellucid area of a chick with eight protovertebræ. In fig. 3 the nuclei are seen to have considerably increased in number at the points of starting of the protoplasmic processes. At _n_ is seen a nucleus with two nucleoli. Fig. 4 is taken from the anterior part of the pellucid area of an embryo of thirty-six hours. It shews the narrow processes characteristic of the anterior part of the pellucid area, and the fewer nuclei. Small spaces, which have the appearance of vacuoles, are shewn at _v_. Fig. 5 is taken from the posterior part of the pellucid area of a thirty-six hours' embryo. It shews the nuclei, with somewhat irregular nucleoli, which have begun to acquire the red colour of blood-corpuscles; the protoplasmic processes containing the nuclei; the nuclei in the protoplasm surrounding the corpuscles, as shewn at _a_, _a´_. Fig. 6 shews fully formed blood-vessels, in part filled with blood-corpuscles and in part empty. The walls of the capillaries, formed of cells, spindle-shaped in section, are shewn, and also the secondary investment of Klein at _k_, and at _b_ is seen a narrow protoplasmic process filled with blood-corpuscles. Fig. 7 is taken from the anterior part of the pellucid area of a thirty-six hours' embryo. It shews a collection of nuclei which are beginning to become blood-corpuscles. Figs. 1-5 are drawn with an 1/8 object-glass. Fig. 6 is on a much smaller scale. Fig. 7 is intermediate. Fig. 8.--A transverse section through the dorsal region of a forty-five hours' embryo; _ao_, aorta with a few blood-corpuscles. v, Blood-vessels, all of them being formed in the splanchnopleure, and all of them provided with the secondary investment of Klein; _pe_, pellucid area; _op_, opaque area. Fig. 9.--Small portion of a section through the opaque area of a thirty-five hours' embryo, showing protoplasmic processes, with nuclei passing from the somatopleure to the splanchnopleure.