t is the lining
of the intestine and of the glands which open into it; and
the material of the notochord is also regarded, as hypoblast.

Section 22. Figure 9 ii. shows all the essential points of the structure
of amphioxus. Epiblast is indicated by a line of dashes, mesoblast
by dots, and hypoblast, dark or black. The true mouth is formed late
by a tucking-in of epiblast, the stomodaeum (s.d.), which meets and
fuses with the hypoblast, and is then perforated. The position of this
mouth is at the velum. The formation of the atrium has been
described. The metapleural folds run forward in front of the velum, as
the epipleurs (ep. in Sections 1 and 2), and form an oral hood (b.c.),
around which the tentacles appear, and which is evidently not
equivalent to the vertebrate mouth cavity, but in front of and outside
it. The anus is formed by a tucking in, the proctodaeum, similar to the
stomodaeum.

Section 23. The formation of the respiratory slits is complicated, and
difficult to describe, but, since investigators have still to render its
meaning apparent, it need not detain the elementary student.*

* See Balfour's Embryology, Volume 2, and Quarterly Journal of
Microscopical Science March, 1891.

_Questions on Amphioxus_

1. Draw diagrams, with the parts named, of the alimentary canal of (a)
amphioxus, (b) any craniate; (c) indicate very shortly the principal
structural differences between the two.

2. Describe, with a diagram, the circulation of amphioxus. Compare it
with that of the craniata.

3. Draw from memory transverse sections, of amphioxus (a) in the
oral region, (b) through the pharynx, (c) just anterior, and (d) just
posterior to atrial pore.

4. Describe fully the coelom of amphioxus, and compare it with that of
the frog in regard to (a) development, (b) its relation to other organs in
the adult.

5. Compare the atrial cavity and coelom of amphioxus. To what series
of cavities in the frog are the metapleural canals to be compared?

6. Describe the notochord of amphioxus, and point out its differences
from the vertebrate notochord.

7. Describe, with diagrams, the nervous system of amphioxus, and
compare its nervous axis, in detail, with that of a vertebrate.

8. Compare the genital organs of amphioxus with those of a higher
vertebrate.

9. What structures have been regarded, as renal organs in
amphioxus?

10. What is a gastrula? With what lower type has the gastrula been
compared? Dis