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.