tadpole,
corresponding to Figure 8 of Amphioxus.

Figures 8 and 9. Diagrammatic longitudinal sections of tadpoles
(compare 9. ii. of Amphioxus). Y. represents a mass of
yolk cells.

Figure 10. Side view of young tadpole, showing external gills (e.g.)
and suckers (s.). Note the ventral bulging due to the yolk.

Figure 11. Ventral view of a later tadpole.
op., the operculum.
int., coiling intestine.

Figure 12. Head of still later tadpole in horizontal section to show
atrial chamber formed by operculum.
int.g., internal gills.
L., developing lungs.

Figure 13. Diagrammatic cross-section of the mid-dorsal part of an
embryonic vertebrate.

ao., aorta.
B.C., Bowman's capsule.
coe., coelom.
d.g., ganglion on dorsal root of spinal nerve.
gl., -its branch- [arteriole] to form glomerulus.
g.r., genital ridge.
I., intestine.
M.D., Mullerian duct.
ns. [nst.], nephrostome.
n.c., notochord; -n.s.-, [n.sh.] its sheath.
s.c., neural canal.
W.D., Wolffian duct.

{Illustration: Diagram Sheet 22.}

Sheet 23.

-The Development of the Fowl_.

Figure 1. Diagram of the early ovum. The section below is a small
portion of the blastodermic area.
b.d., blastoderm.
y., the undivided yolk.
s.c., the segmentation between the blastoderm and yolk. Compare
s.c. in {Sheet} 22, {Figure} 4.

Figure 2. Area pellucida about the sixteenth hour. The figure below is
the central part of the section indicated by the transverse line, and
showing the primitive streak (p.s.).

Figure 3. Area pellucida about the twenty-first hour. Two sections
through a and b below.

Figure 4. About the twenty-fifth hour; surface view; longitudinal section
to right and transverse above.

Figure 4b. Diagrammatic rendering of same stage (compare Figure 9
of Frog and 9.ii. Amphioxus). This will be most clearly understood if
the reader look at Sheet 22, {Figure} 9, and imagine Y. enormously
increased, and the embryo sinking into it. Epiblast, ep., -line of
dashes- [black line]. Mesoblast, dotted. Hypoblast, -black- [line of
dashes]. pp., the pleuro-peritoneal cavity.

Figure 5 and 6 illustrate formation of amnion (a.) and allantois (all.).
6 is about the fourth day.

{Illustration: Diagram Sheet 23.}

Sheet 24.

-The Development of the Fowl_.
Figure 1. Chick about the -fifth- [third] day. At this stage the chick lies
on its left side in the yolk. [For lettering of blood vessels, see (7)
below.]
i., the intestine.
u.v., the yolk sac.