- 96 - 
an illustration of a longitudinally transsected nerve, drawn under 
higher powers than fig. 19. The nerve-tubes are, however, narrower 
in fig. 20, than they are in fig. 19 (cmfi-. a! and 6').^) 
The reason why the longitudinal neuroglia fibres in the nerves and 
pedal nerve-cords are isolated in macerated preparations is, evidently, 
that they are stouter and stronger than the rest of the tube-sheaths, 
and thus are separated from them. In carefully treated macerated 
preparations it is, however, possible to see the nerve-tubes. Fig. 17 
and 18 represent, for instance, such preparations, where nerve-tubes 
with a longitudinal striation are distinctly visible, and even to some 
extent isolated ; an isolation of them is, however, as a rule, extremely 
difficult, for the reason just indicated above. In the extremities o 
these nerve-tubes v\^e can see, in some of them, isolated fibrillæ, 
which, partly, are neuroglia fibrillæ, partly, spongioplasmic fibrillæ. 
Pearls of hyaloplasm are adherent to the sides of them (fig. 17, a, h, d). 
The Åscidians. 
Finally, I will here mention a group of invertebrates which some 
time ago were the subject of my study, the results of which I have 
hitherto only given in a preliminary report (1. c. 1886). This 
group is the Åscidians, The peripheral nerves of the Åscidians 
have a structure very similar to what is described of the peripheral 
nerves of Homarus (cmfr. fig. 7), exept as regards the large tubes; 
those do not occur in the Ascidian-nerves. 
The whole nerve is, in the Åscidians, divided by the neuroglia, 
or inner neurilem, into large bundles, these are again divided into 
smaller and these, again, into still smaller bundles, the subdivision being 
repeated until we at last arrive at the nerve-tubes, which are very 
slender, and contain but few primitive tubes; a great many nerve- 
tubes seem to consist, even, of only one primitive tube, something 
similar to what we have described in Homarus. Very often, it is 
even very difficult to decide what are only primitive tubes, and what 
are nerve-tubes, and again what are bundles of nerve-tubes, as there 
is often seen, in transverse sections, a subdivision into smaller and 
smaller tubes, and, the higher and higher we employ the powers of 
the microscope the more and more do we trace out minute tubes. 
At last, however, under very high powers we are able to observe 
^) Sometimes large vacuoles are seen in transverse section of nerves (cmfr. 
fig. 19, C, d); whether these vacuoles really are transsected nerve-tubes I am not 
in a position to decide at present. 
