422 MONTGOMERY. [Vol. XIII. 



core and courses towards the posterior end of the chord, where 

 it becomes gradually indistinguishable, owing to decrease in 

 diameter. On cross section it is more or less circular or oval 

 in outline, and is bounded by a fine sheath; its contents do not 

 stain and are homogeneous, except that occasionally vestiges of 

 a very fine, reticular structure may be found within it. On 

 account of its large size and general appearance, especially on 

 longitudinal sections, it bears a close resemblance to a neuro- 

 chord; but neurochord cells are absent in Lineus. This struc- 

 ture certainly can represent nothing else than a bundle of nerve 

 tubules, and accordingly may be termed a nerve tube ; the fine 

 reticulation which its contents sometimes show would corre- 

 spond to the sheaths of the individual nerve tubules composing 

 it. It is the " Faserstrang " found by Burger ('90b, '91b), but 

 it is not composed of a bundle of " nerve fibrils." The fine 

 membrane surrounding the nerve tube is so minute that I can- 

 not determine whether it is a neuroglia product sui generis, or 

 whether it is formed by a coalescence of the sheaths of the 

 outermost layer of enclosed nerve tubules. This tube is not 

 continued, anteriorly, into the ventral lobe of the brain, nor 

 into the first commissure of the latter. Further, it does not 

 persist as a single, unbranched tube through the whole length 

 of the lateral chord. For on following a series of cross sec- 

 tions, the first section may show a single tube, the second two 

 tubes (each half the diameter of that on the foregoing section), 

 further sections may show three or four tubes, and a consequent 

 section the disc of but one tube again (Fig. 33 a-d shows the 

 branching of the nerve tube as seen on four consecutive cross 

 sections). That the tube branches, apparently dichotomously, 

 can be observed on longitudinal sections also. Thus the nerve 

 tubules in the nerve tube do not pass singly out of the latter 

 into the peripheral nervous system, but smaller tubes (each 

 containing a number of nerve tubules) branch off from the 

 main tube. It is possible that towards the posterior end of 

 the lateral chord, where the diameter of the main nerve tube 

 becomes much reduced, the nerve tubules pass out of it singly; 

 but I do not attempt to decide this point, which has no direct 

 bearing upon the questions at issue. 



