Part 3

Figs. 6 and 7 are sections through an embryo rather earlier than the one drawn in fig. 8. Fig. 6 passes through the just commencing medullary groove (md), which appears in fresh specimens, as in fig. 8, merely as an opaque streak coming from the end of the primitive groove. The notochord is hardly differentiated, but the complete separation of mesoblast and hypoblast under the primitive groove is clearly shewn. Fig. 7 passes through the anterior end of the primitive groove (pr), and shews the fusion between the mesoblast and epiblast, which is always to be found under the primitive groove.

Fig. 8 is a view from above of a twenty hours' blastoderm, seen as a transparent object. Primitive groove (pr). Medullary groove (md), which passes off from the anterior end of the primitive groove, and is produced by the thickening of the mesoblast. Head fold (pf).

Figs. 13-17 are sections through the blastoderm, drawn in fig. 18 through the lines 1, 2, 3, 4, 5 respectively.

The first section (fig. 13) passes through the true medullary groove (mc); the two medullary folds (A, A) are seen on each side with the thickened mesoblast, and the mesoblast cells are beginning to form the notochord (nc) under the medullary groove. There is no adherence between the mesoblast cells and the epiblast under the medullary groove.

The second (fig. 14) section passes through the medullary groove where it has become wider. Medullary folds, A, A; notochord, ch.

In the third section (fig. 15) the notochord (ch) is broader, and the epiblast is raised in the centre, while the medullary folds are seen far apart at A.

In section fig. 16 the medullary folds (A) are still to be seen enclosing the anterior end of the primitive groove (pr). Where the primitive groove appears there is a fusion of the epiblast and mesoblast, and no appearance of the notochord.

In the last section, fig. 17, no trace is to be seen of the medullary folds.

Figs. 18 and 19 are magnified views of two hardened blastoderms. Fig. 18 is twenty-three hours old; fig. 19 twenty-five hours. They both shew how the medullary canal arises entirely independently of the primitive groove and in front of it, and also how the primitive groove gets pushed backwards by the growth of the medullary groove. pv, Protovertebræ; other references as above. Fig. 18 is the blastoderm from which sections figs. 13-17 were cut.

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.

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.

Fig. 10. Section through the heart of a thirty-four hours' embryo. a. Alimentary canal; hb, hind brain; nc, notochord; e, epiblast; so, mesoblast of the somatopleure; sp, mesoblast of the splanchnopleure; hy, hypoblast; hz, cavity of the heart.

Fig. 11. Section through the same embryo as fig. 10, and passing through the orifice of the omphalomeseraic vein. of, Omphalomeseraic vein; other references as above.

These two sections shew that the heart is entirely formed from the mesoblast of the splanchnopleure, and that it is formed by the splitting of that part of the mesoblast which has turned to assume its normal direction after being folded in to form the muscular wall of the alimentary canal. In fig. 11 the cavities so formed on each side have not yet united, but in fig. 10 they have united. When the folding becomes more complete the cavities (of, of) in fig. 11 will unite, and in this way the origin of the omphalomeseraic veins will be carried further backwards. In the section immediately behind section 11 the mesoblast had become thickened, but had not split.

EXPLANATION OF PLATES 3 AND 4. (V. p. 60.)

COMPLETE LIST OF REFERENCE LETTERS.

al. Alimentary canal. ao. Dorsal aorta. auv. Auditory vesicle. bd. Formative cell probably derived from the yolk. cav. Cardinal vein. ch. Notochord. ch´. Thickening of hypoblast to form the notochord. eb. Line indicating the edge of the blastoderm. ep. Epiblast. ep´. Epidermis. er. Embryonic rim. es. Embryonic swelling. gl. Glosso-pharyngeal nerve. h. Head. ht. Heart. hy. Hypoblast. ll. Lower layer cells. ly. Line of separation between the blastoderm and the yolk. m. Mesoblast. mc. Medullary canal. mg. Medullary groove. mp. Muscle-plate. mp´. Early formed mass of muscles. n. Peculiar nuclei formed in the yolk. n´. Similar nuclei in the cells of the blastoderm. na. Cells which help to close in the alimentary canal, and which are derived from the yolk. ny. Network of lines present in the food-yolk. ol. Olfactory pit. op. Eye. ov. Oviduct. pn. Pineal gland. pov. Projection which becomes the ovary. pp. Pleuro-peritoneal cavity. pp´. Remains of pleuro-peritoneal cavity in the head. prv. Protovertebræ. pwd. Primary points of involution from the pleuro-peritoneal cavity by the coalescence of which the Wolffian duct is formed. sg. Segmentation cavity. so. Somatopleure. sos. Stalk connecting embryo with yolk-sac. sp. Splanchnopleure. spn. Spinal nerve. sur. Suprarenal body. ts. Caudal lobes. v. Blood-vessel. vg. Vagus nerve. V. Fifth nerve. VII. Seventh nerve. vc, 1, 2, 3, &c. 1st, 2nd and 3rd &c. visceral clefts. vp. Vertebral plates. wd. Wolffian duct. x. Peculiar body underlying the notochord derived from the hypoblast. yk. Yolk spherules.

All the figures were drawn with the Camera Lucida.

PLATE 3.

Fig. 1. Section parallel with the long axis of the embryo through a blastoderm, in which the floor of the segmentation cavity (sg) is not yet completely lined by cells. The roof of the segmentation cavity is broken. (Magnified 60 diam.) The section is intended chiefly to illustrate the distribution of nuclei (n) in the yolk under the blastoderm. One of the chief points to be noticed in their distribution is the fact that they form almost a complete layer under the floor of the segmentation cavity. This probably indicates that the cells whose nuclei they become take some share in forming the layer of cells which subsequently (vide fig. 4) forms the floor of the cavity.