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).

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.