Part 6

Figs. 8a, 8b, 8c. Modified nuclei from the yolk from the same germinal disc.

Fig. 8d. Segment in the act of division from the same germinal disc.

Fig. 9. Section through a germinal disc in which the segmentation is completed. It shews the larger collection of cells at the embryonic end of the germinal disc than at the non-embryonic. ep. epiblast.

EXPLANATION OF PLATE 7. (X. p. 246.)

COMPLETE LIST OF REFERENCE LETTERS.

c. Cells formed in the yolk around the nuclei of the yolk. ep. Epiblast. er. Embryonic ring. es. Embryo swelling. hy. Hypoblast. ll. Lower layer cells. ly. Line separating the yolk from the blastoderm. m. Mesoblast. mg. Medullary groove. n´. Nuclei of yolk. na. Cells to form ventral wall of alimentary canal which have been derived from the yolk. nal. Cells formed around the nuclei of the yolk which have entered the hypoblast. sc. Segmentation cavity. vp. Combined lateral and vertebral plate of mesoblast.

Fig. 1. Longitudinal section of a blastoderm at the first appearance of the segmentation cavity.

Fig. 2. Longitudinal section through a blastoderm after the layer of cells has disappeared from the floor of the segmentation cavity. bd. Large cell resting on the yolk, probably remaining over from the later periods of segmentation. Magnified 60 diameters. (Hardened in chromic acid.)

The section is intended to illustrate the fact that the nuclei form a layer in the yolk under the floor of the segmentation cavity. The roof of the segmentation cavity is broken.

Fig. 2a. Portion of same blastoderm highly magnified, to shew the characters of the nuclei of the yolk n´ and the nuclei in the cells of the blastoderm.

Fig. 2b. Large knobbed nucleus from the same blastoderm, very highly magnified.

Fig. 2c. Nucleus of yolk from the same blastoderm.

Fig. 3. Longitudinal section of blastoderm of same stage as fig. 2. (Hardened in chromic acid.)

Fig. 4. Longitudinal section of blastoderm slightly older than fig. 2. Magnified 45 diameters. (Hardened in osmic acid.)

It illustrates (1) the characters of the epiblast; (2) the embryonic swelling; (3) the segmentation cavity.

Fig. 5. Longitudinal section through a blastoderm at the time of the first appearance of the embryonic rim, and before the formation of the medullary groove. Magnified 45 diameters.

Fig. 5a. Section through the periphery of the embryonic rim of the blastoderm of which fig. 5 represents a section.

Fig. 6. Section through the embryonic rim of a blastoderm somewhat younger than that represented on Pl. 8, fig. B.

Fig. 7. Section through the most projecting portion of the embryonic rim of a blastoderm of the same age as that represented on Pl. 8, fig. B. The section is drawn on a very considerably smaller scale than that on fig. 5. It is intended to illustrate the growth of the embryonic rim and the disappearance of the segmentation cavity.

Fig. 7a. Section through peripheral portion of the embryonic rim of the same blastoderm, highly magnified. It specially illustrates the formation of a cell (c) around a nucleus in the yolk. The nuclei of the blastoderm have been inaccurately rendered by the artist.

Figs. 8a, 8b, 8c. Three sections of the same embryo. Inserted mainly to illustrate the formation of the mesoblast as two independent lateral masses of cells; only half of each section is represented. 8a is the most posterior of the three sections. In it the mesoblast forms a large mass on each side, imperfectly separated from the hypoblast. In 8b, from the anterior part of the embryo, the main mass of mesoblast is far smaller, and only forms a cap to the hypoblast at the highest point of the medullary fold. In 8c a cap of mesoblast is present, similar to that in 8b, though much smaller. The sections of these embryos were somewhat oblique, and it has unfortunately happened that while in 8a one side is represented, in 8b and 8c the other side is figured, had it not been for this the sections 8b and 8c would have been considerably longer than 8a.

Fig. 9. Longitudinal section of an embryo belonging to a slightly later stage than B.

This section passes through one of the medullary folds. It illustrates the continuity of the hypoblast with the remaining lower layer cells of the blastoderm.

Figs. 10a, 10b, 10c. Three sections of the same embryo belonging to a stage slightly later than B, Pl. 8. The space between the mesoblast and the hypoblast has been made considerably too great in the figures of the three sections.

10a. The most posterior of the three sections. It shews the posterior flatness of the medullary groove and the two isolated vertebral plates.

10b. This section is taken from the anterior part of the same embryo and shews the deep medullary groove and the commencing formation of the ventral wall of the alimentary canal from the nuclei of the yolk.

10c shews the disappearance of the medullary groove and the thinning out of the mesoblast plates in the region of the head.

Fig. 11. Small portion of the blastoderm and the subjacent yolk of an embryo at the time of the first appearance of the medullary groove × 300. It shews two large nuclei of the yolk (n) and the protoplasmic network in the yolk between them; the network is seen to be closer round the nuclei than in the intervening space. There are no areas representing cells around the nuclei.