On the Structure of the Excretory Organs of Amphioxus. Part 2.—The ...

STRUCTURE OF THE EXCRETORY ORGANS OF AMPHIOXUS. 187
canal; but this deceptive appearance is soon exposed on a
more critical examination of the preparation. Thick sections
are especially misleading. No observation made on a section
more than 5 ju thick is in the least conclusive. The technical
difficulties are very great in the study of Amphioxus;
the tissues are brittle, the cells very small and difficult
to stain satisfactorily. Formol and Flemming's fluid, corrosive-acetic,
and picro-sulphuric-formol are all good preservatives.
Great care must, however, be taken to avoid
shrinkage, and for this purpose the method of double
embedding in celloidin and paraffin is most useful. By far
the best sections are obtained from pieces of the pharynx
removed from the fresh animal, and preserved separately.
One may use either carmine or hrematoxylin for staining the
nuclei; but it is quite essential to add some suitable cytoplasmic
stain such as acid fuchsin. For the particular
purpose we are now concerned with, perhaps some strong
staining reagent like Mann's methyl-blue eosin is the best
for working' out minute details under high powers, though
picro-nigrosin also yields valuable results.
Turning now to the structure of the nephridium, we find
the external pore opening at the very top of the atrial cavity,
on the anterior outer surface of the secondary or tongue bar
{of., figs. 1, 2, 7, and text-figure). The pore leads into
a canal which gives off a short posterior limb, and a much
longer anterior limb. The latter passes forwards to the next
primary bar, and downwards into the triangular ccelomic
cavity delimited by the ligamentum denticulatum. In a
fully developed nephridium both the anterior and posterior
limbs give off diverticula of varying length, which may
sometimes branch. These are shown in fig. 1 of Part 1 (7),
and are seen again in the reconstructions given in this paper
(figs. 1, 2, 3).
Let us pass to the conclusive evidence which can only be
obtained from sections. The wall of the nephridial canal
contains many nuclei (figs. 7, 13). In some places they are
so closely packed that they seem to press against each other.