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.