— ISO — 
thelium of cylindrical cells, and outside this epithelium a limited 
number of ganglion cells (vide fig. 90). The rest of the spinal cord 
consists of white substance. 
The white sub stance and the nerve-tub es. — As is easily 
seen in transverse sections, the white substance of Amphioxus con- 
sists, chiefly, of longitudinal (partly also transverse) nerve-tubes, which 
in transverse sections are transsected and give the section the ap- 
pearance of containing a reticulation (vide fig. 90). Some of the 
longitudinal nerve-tubes are very large, and very much resemble, in 
their appearance, the large nerve-tubes which are, for instance, 
present in the ventral nerve-cord of Nereis. They are especially 
situated in the lateral ventral parts of the white substance (vide fig. 90). 
The large tubes, which OwsjANNlKOW has described as blood-vessels, 
are colossal nerve-tubes. There is one colossal nerve-tube running 
on the ventral side of the nerve-cord, just under the central canal 
(fig. 90, vnt). One colossal nerve-tube generally runs, laterally, in 
each side of the spinal cord (fig. 90, Int). These three nerve-tubes, 
especially the ventral one, are so far as my experiece goes, the 
largest ones existing in the nervous system of Amphioxus. 
The small nerve-tubes vary very much in size, and some of 
them have an extremely small diameter. 
There are no blood-vessels present in the spinal nerve-cord. 
The white substance is traversed by a great many radiating 
fibres, issuing from the cylindrical cells of the epithelium investing 
the central groove, and passing to the sheath enveloping the spinal 
cord, with which sheath they unite. 
The structure of the nerve-tubes. — The nerve-tubes have 
a structure, quite similar to what is found in the invertebrates we 
have before examined. They consist of a rather deeply staining 
sheath and a less staining contents. The sheaths are generally very 
thin. The contents consist of primitive tubes, quite similar to those 
which are, for instance, present in the nerve-tubes of Homarus. 
The primitive tubes may easily be seen in the large nerve-tubes 
(vide fig. 91); the spongioplasmic walls separating the tubes contain- 
ing hyaloplasm are here very distinct. 
The ganglion cells, — The ganglion cells occur rather spar- 
ingly. They are situated on both sides of the central groove outside 
the epithelium. Some ganglion cells are very large (vide fig. 90, gc) 
but generally they have a moderate size. They are usually multi- 
polar, have always only a single nervous process, the other processes 
are protoplasmic ones. The protoplasmic processes seem generally 
