From the epiblast the epidermis (not the dermis), the nervous system (including the nerves), and the sensory part of all sense organs are derived. From the mesoblast the muscles, the dermis genital and excretory organs, circulatory fluid and apparatus, any skeletal structures; and all connective tissue are derived. The mass of the body is thus evidently made of mesoblast. The hypoblast 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.
3. _Questions on Amphioxus_
- 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.
- Describe, with a diagram, the circulation of amphioxus. Compare it
with that of the craniata.
- 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.
- 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.
- Compare the atrial cavity and coelom of amphioxus. To what series
of cavities in the frog are the metapleural canals to be compared?
- Describe the notochord of amphioxus, and point out its differences
from the vertebrate notochord.
- Describe, with diagrams, the nervous system of amphioxus, and
compare its nervous axis, in detail, with that of a vertebrate.
- Compare the genital organs of amphioxus with those of a higher
vertebrate.
- What structures have been regarded, as renal organs in
amphioxus?
- What is a gastrula? With what lower type has the gastrula been compared? Discuss the comparison.