_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. 2_a_. 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. 2_b_. Large knobbed nucleus from the same blastoderm, very highly
magnified.
Fig. 2_c_. 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. 5_a_. 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. 7_a_. 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. 8_a_, 8_b_, 8_c_. 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. 8_a_ 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 8_b_, 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 8_c_ a cap of mesoblast is present, similar to that in 8_b_,
though much smaller. The sections of these embryos were somewhat oblique,
and it has unfortunately happened that while in 8_a_ one side is
represented, in 8_b_ and 8_c_ the other side is figured, had it not been
for this the sections 8_b_ and 8_c_ would have been considerably longer
than 8_a_.
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. 10_a_, 10_b_, 10_c_. 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.
10_a_. The most posterior of the three sections. It shews
the posterior flatness of the medullary groove and the two
isolated vertebral plates.
10_b_. 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.
10_c_ 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.
Fig. 12. Nucleus of the yolk in connection with the protoplasmic network
hardened in osmic acid.
Fig. 13. Portion of posterior end of a blastoderm of stage B, shewing the
formation of cells around the nuclei of the yolk.
Fig. 14. Section through part of a young Scyllium egg, about 1/15th of an
inch in diameter.
_nl._ Protoplasmic network in yolk. _zp._ Zona pellucida. _ch._
Structureless chorion. _fep._ Follicular epithelium. _x._ Structureless
membrane external to this.