— 82 — 
manner that the intervals between the lines are narrower in the 
centre than they are more peripherically, and this is all the more 
prominent the narrower the axis is. 
This striation in some large nerve-tubes is already observed and 
described by Remak, and subsequently to him a great many other 
writers have described it, even Hæckel, who describes the tube- 
contents as being homogeneous, has seen it (as mentioned p. 30 
foot note 4) in some tubes. FREUD assures us, however, that a 
similar striation is visible in all nerve-tubes in a quite fresh state. 
In spite of this statement of Freuds and in spite of the application 
of the best lenses (Zeisss new apochr. lenses) I must confess that, 
I have found it extremely difficult to observe a striation in a great 
many nerve-tubes. In the greatest number of the slender tubes 
and tubes of middle size, and even in some large tubes, it required, 
certainly, a predisposition to see a striation, and I really think that 
an uninterested eye would, in many cases, see no striation, even 
when the nerves or commissures were instantly tåken from a quite 
fresh and living animal, which FREUD states to be a quite neces- 
sary condition if a striation is to be seen in all tubes. I have 
also, in some rather slender tubes, observed a but little prominent 
striation (fig. 12, b) but there are a great many tubes and, as men- 
tioned, even large ones which have left me very doubtful as to 
their striation in the fresh state. 
I do not think this, however, to be decisive as to the fibrillar 
or non-fibrillar structure of these tubes in the live-state, because, a 
priori, I think it to be very probable that if there are two sub- 
stances in the tubes, their refractive difference is perhaps so small 
that, when one of the substances is present only in very slender 
filaments or membranes, the whole contents look as a homogeneous 
mass, and that the refractive difference must be altered by chemical 
(fixing) agents if the substances are to become easily visible. That 
such is the case, and that a distinct striation becomes visible in all 
nerve-tubes when treated with various fixing agents (chromic acid, 
osmic acid, picric acid, acetic acid, nitric acid, potassium-bichromate, 
etc. etc.) is a so well-known fact that it, certainly, need not be 
further mentioned here. (Pl. II, fig. 15 & 16 epresents some smaller 
nerve-tubes; macerated for some hours in glycerine, acetic acid and 
water; a striation was distinctly visible.) 
Almost all writers who have seen a striation in the tubes in 
the live-state have agreed in declaring the darker thin lines of the 
striation to be nervous fibrillæ (»Primitivfibrillen«) swimming in an 
