the
atrial pore to the atrial cavity. Note coe., the body cavity.

References to the two figures.
an., anus.
at., atrial cavity.
at.w., atrial wall.
at.p., atrial pore.
a.d., anterior dilatata of nervous system.
b.w., body-wall.
b.t.L., brown tubes of Lankester.
c.f., ciliated funnel.
coe., coelome.
c.ao., cardiac aorta.
d.ao., dorsal aorta (paired).
d.ao'., dorsal aorta median.
g., gonads (male or female genital gland).
hep., hepatic vein.
in., intestine.
i.w., intestine wall.
lv., liver.
m.f., median fin.
n.c., notochord.
p.v., portal vein.
ph., pharynx.
-p.s.-, [e.s.] pigment spot ("eye spot").
s.c., spinal cord.

{Illustration: Diagram Sheet 19.}

Sheet 20

-Sections of Amphioxus_.

The Roman numerals indicate the corresponding region in Figure 1,
Sheet 19. The lettering is identical; but note, in addition;
br.c., branchial canal.
c.f., ciliated funnel.
d.c.c., dorsal coelomic canal.
end., endostyle.
ep., epipleur.
e.s., eye spot.
h.p., hypopharyngeal grove.
h.vn., for hepatic vein.
o.c., oral cavity (or hood).

{Illustration: Diagram Sheet 20.}

Sheet 21.

-Phases in the Development of Amphioxus_.

Figures 1, 2, 3, 4. Phases in segmentation.

Figure 5. The blastosphere.

Figure 6. The gastrula in section, anterior end to the right.

Figure 7. i. Dorsal view post gastrula stage.

Figure 7. ii. Diagrammatic section of the same in the position
indicated by the transverse line in 7, i.

Figure 8. Diagrammatic section of a later stage.
coe.p., the coelomic pouches.
n.c., the notochord.
n.p., the neural plate.

Figure 9.i. Still later section.

Figure 9. ii. Diagrammatic view of late embryo.

Figures 10, 11, 12 illustrate the formation of the atrium as a median
ventral invagination, at.

{Illustration: Diagram Sheet 21.}

Sheet 22.

-The Development of the Frog_.

These diagrams must be studied with the text. They should be
compared with the corresponding ones of Amphioxus as indicated
below.

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