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stouter neurilem-sheaths, and so on, concentrically, untill at last the 
whole nerve is enclosed in one common external netirilem-sheath.^) 
It is, indeed, very diflicult to tell here where the separation of indi- 
vidual nerve-tubes really begins, if we do not, as I have done, con- 
sider the smallest tubes with distinct stout sheaths as representing, 
each of them, a nerve-tube, consisting of some few primitive tubes 
only, or, in a great many cases, of one primitive tube only. 
When we examine a transverse section of a nerve, we will, as 
a rule, observe a great many dark granules or dots; these dots, 
appear, however, on closer examination, to be thickenings in the 
tube-sheaths along the longitudinal edges where several tubes meet 
(vide fig. 8, p). 
The sheaths of the nerve-tubes. — Our attention has 
hitherto been directed to the contents of the nerve-tubes only. We 
will, now, before we leave the nerve-tubes of the lobster, pay a 
little attention to the sheath which envelopes the contents. This 
sheath consists of a connective substance which is, in my opinion, 
the same substance as the nenroglia, or »connnctive-tissue«, as many 
authors call it, which extends through the whole central nervous 
system of every animal I have had under investigation. This 
sheath of connective substance, or for the sake of brevity we 
will call it neiiroglia-sheath, has, also, a great resemblance to the 
spongioplasm separating the primitive tubes, and it is, in fact, very 
often, extremely difficult to distinguish the two substances from each 
other viz. the spongioplasm occurring inside the nerve-tubes and the 
ganglion cells — as will subsequently be described — and the 
neuroglia enveloping the nerve-tubes and the ganglion cells with 
sheaths or membranes; the two substances are often so intimately 
united that it is really impossible to decide where the line of de- 
marcation can be drawn. There is one difference, however, viz. that 
in the spongioplasm no nuclei occur, whilst nuclei occur in the 
neuroglia. Leydigs view that it is the same substance, spongioplasm, 
which penetrates from whithout into the nerve-system and even into 
the nervous elements, is, in my opinion, not yet sufficiently well sub- 
stantiated. If I had to choose, I would, however, much prefer that 
theory to Vejdovskys, according to which the inner connective 
substance (i. e. neuroglia) is a product originating in the ganglion 
cells; I really do not understand how, in that case, to account for 
') This separation into bundles and fagets is, however, les prominent or not 
present at all near the origin of the nerves in the central nerve-system. 
