Sheet 22.
-The Development of the Frog_.

Figures 1, 2, 3. Stages in segmentation
(compare 1, 2 ,3 of {[Sheet 21]} Amphioxus).
Figure 4. Blastosphere stage (compare [5], Amphioxus). This, on a
smaller scale. The cells on the ventral side are so much larger
because distended with yolk.
Figure 5. Gastrula stage in section (compare [6], Amphioxus). The
Frog on a smaller scale than Amphioxus.
Figure 6. Dorsal view of gastrula (compare [7], Amphioxus).
Figure 7. Part of a transverse section of developing 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.

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.