EXPLANATION OF [PLATE 2]. (IV. p. 47.)
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.