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the neuroglia-cells and nuclei, and how they can be produced from 
ganglion cells. 
In my opinion there can be no doubt, but that the neuroglia is 
a separate tissue composed of cells springing from the ectoderm, 
just as the ganglion cells spring from other ectodermal cells. 
The doubly marked outlines which are visible in nerve-tubes 
isolated in the fresh state, but which are specially distinct in macerated 
nerve-tubes, are produced by the sheath which, as a cylinder, enve- 
lopes every nerve-tube and forms its outer isolating layer. It con- 
sists of one or several concentric layers of connective substance or, 
as we just above called it, neuroglia, and is easily seen in transverse 
as well as in longitudinal sections; the sheaths of large nerve-tubes 
are especially very prominent and stout. When there are several 
layers; which is generally the case in large nerve-tubes, especially 
those of the peripheral nerves; then the innermost layer is the 
strongest, most dififerentiated, and refractive one (vide fig. 15). These 
concentric layers of the tube-sheaths are seen, in fig. 7, round the 
large nerve-tubes {t), and in some peripheral nerves they are very 
prominent. 
In the sheaths, nuclei occur. These nuclei are, as mentioned, 
quite identical with the usual nuclei of the neuroglia; they have an 
oblong form, with a granular appearance, and are usually situated on 
the outer side of the sheaths (vide fig. 2, fig. 5, Tc\ fig. 8); they 
occur, however, also on the inside of the sheath, consequently in 
the tube itself (vide fig. 4, k\ fig. 5, A;'; fig. 13, k). 
Nephrop norvegicus. 
We obtain very similar results as to the structure of the nerve- 
tubes on examining Nephrops norvegicus; in this respect Nephrops 
so very much resembles Homarus that it is really unnecessary to 
give any special description of it. We can, in fact, observe, in the 
large nerve-tubes, the same tendency towards a concentration of a 
sort of axis in their centre, though it is not so prominent in Nephrops 
as in Homarus, and neither have I observed so narrow and deeply 
stained axes in the former as I have in the latter. 
In the transverse sections of many nerve-tubes, I have observed 
similar large meshes as those. I have mentioned in the nerve-tubes 
of Homarus (cmfr. fig. 5, t^). They had often a very regular ap- 
pearance and looked as if they really were large transsected primi- 
tive tubes. 
