No. 3-] STUDIES ON THE HETERONEMERTINI. 419 



sheath of Schwann {S. Sh), the separate longitudinally directed 

 fibres of which are derived from the fibrous sheath of the cell 

 itself. As far as I could determine, the sheath of Schwann 

 consists of a single layer of longitudinal neuroglia fibres, the 

 fibres being parallel to, but not in contact with, one another, i.e., 

 they apparently do not produce a continuous membrane. I 

 could not determine along how great an extent of the nerve tubule 

 this sheath continues, since its fibres decrease in size distally. 

 Certainly, however, in its proximal portion the following com- 

 ponents of the nerve tubule may be distinguished: (i) the axial, 

 hyaloplasmic axis cylinder, (2) the spongioplasmic sheath of 

 the latter, and (3) on the outside of this sheath, the sheath 

 of Schwann. On a distal portion of the nerve tubule the axis 

 cylinder is found to be bounded by a very fine, scarcely visible 

 envelope (Fig. 40); the latter is so minute that one cannot 

 determine whether it corresponds to the spongioplasmic and 

 the neuroglia sheath, or whether to merely one of the latter. 



On longitudinal sections of the neurochord nerve tubules 

 (Fig. 42 a-e) the separate fibres of the sheath of Schwann are 

 mostly found to be bisected; this fact might be explained on the 

 ground that they describe a spiral course around the nerve tubule. 



As just described, the sheath of Schwann is formed by a 

 continuation along the nerve tubule of certain of those outer 

 neuroglia fibres which produce the loose sheath around the 

 proximal portion of the ganglion cell. But whether fibres from 

 the cells of the inner neuroglia also take part in the construction 

 of the sheath of Schwann, I have been unable to decide; this 

 might appear probable in those cases where the distal pole of 

 the ganglion cell lies close to the layer of the inner neuroglia 

 cells. Further, I cannot state positively that an additional 

 sheath, derived from the inner neurilemma, is added to the 

 nerve tubule; though in support of there being such a sheath 

 around at least the proximal portion of the tubule, I may refer 

 to the left-hand cell portrayed in Fig. 29, where a thin plate of 

 the inner neurilemma is split off from the latter, and accom- 

 panies the proximal portion of the nerve tubule for a short dis- 

 tance. This was the only case in which I found evidences of 

 such a neurilemmatic sheath. 



