Fig. 1. Section through an unincubated blastoderm, shewing the upper layer,
composed of a single row of columnar cells, and the lower layer, composed
of several rows of rounded cells in which no nucleus is visible. Some of
the "formative cells," at the bottom of the segmentation cavity, are seen
at (_b_).
Fig. 2. Section through the periphery of an eight hours' blastoderm,
shewing the epiblast (_p_), the hypoblast (_h_), and the mesoblast
commencing to be formed (_c_), partly by lower-layer cells enclosed between
the epiblast and hypoblast, and partly by formative cells. Formative cells
at the bottom of the segmentation cavity are seen at _b_. At _s_ is one of
the side folds parallel to the primitive groove.
Fig. 3. Portion of the hypoblast of a thirteen hours' blastoderm, treated
with silver nitrate, shewing the great variation in the size of the cells
at this period. An hour-glass shaped nucleus is seen at _a_.
Fig. 4. Periphery of a twenty-three hours' blastoderm, shewing cell for
cell the junction between the hypoblast (_h_) and white-yolk spheres (_w_).
Fig. 5. Junction between the white-yolk spheres and the hypoblast cells at
the passage from the area pellucida to the area opaca. The specimen was
treated with silver nitrate to bring out the shape of the cells. The line
of junction between the opaque and pellucid areas passes diagonally.
Fig. 9. Section through the primitive streak of an eight hours' blastoderm.
The specimen shews the mesoblast very much thickened in the immediate
neighbourhood of the primitive streak, but hardly formed at all on each
side of the streak. It also shews the primitive groove just beginning to be
formed (_pr_), and the fusion between the epiblast and the mesoblast under
the primitive groove. The hypoblast is completely formed in the central
part of the blastoderm. At _f_ is seen one of the side folds parallel to
the primitive groove. Its depth has been increased by the action of the
chromic acid.
Fig. 10. Hypoblast cells from the hinder end of a thirty-six hours' embryo,
treated with silver nitrate, shewing the regularity and elongated shape of
the cells over the embryo and the smaller cells on each side.
Fig. 11. Epiblast cells from an unincubated blastoderm, treated with silver
nitrate, shewing the regular hexagonal shape of the cells and the small
spherules they contain.
Fig. 12. Portion of the epiblast of a thirty-six hours' embryo, treated
with silver nitrate, shewing the small rounded cells frequently found at
the meeting-points of several larger cells which are characteristic of the
upper layer.