156 bulletin: museum of compauative zoology. 



Tliis and the trunk from the left liorn continue parallel through the 

 mesocoele to its posterior limit (Fig. A). Here they unite, and the 

 resulting Reissner's fibre enters the nervous tissue of the downfolded 

 posterior portion of the mesencephalic roof. The fibre passes through 

 this portion of the brain and the basal part of the cerebellum in thu 

 median plane a little dorsal to the passage connecting the third and 

 fourth ventricles. Where the fibre enters and emerges from the ner- 

 vous tissue, each ventricle is continued into a minute recessus, which 

 is prolonged canal-like for a short distance. 



C. Critical Discussion. 



In both the transverse and sagittal sections that I have studied 

 Reissner's fibre has been found always to enter th(>. riyhf tuberculum 

 acusticum (Fig. A). No trace of it has been found in the left acusti- 

 cuin. Appearances in the adult bear out the developmental evidence 

 that in late larval life the fibre becomes surrounded and enclosed by the 

 developing nerve tissues. In its course through the base of the cerebel- 

 lum and right acusticum the fibre gives oif no collaterals, and is not in 

 direct connection with the nervous elements of these parts. It is, how- 

 ever, probable that this passage of the fibre through the nervo substance 

 is, as we sliall see later, of {)hylogenetic significance, representing a more 

 ancestral position of this fibre tract. 



Sanders ('94, p. 11) described in Myxine the course of Reissner's 

 fibre, wiiich he says is lost in the aqueduct of Sylvius. As his de- 

 scription is not wholly clear to me, I will (piote liis words: "The 

 central canal, at first [i. e., at its posterior end] is single, but pro- 

 ceeding forward it shortly divides into two canals, an upper and a 

 lower. ... At this point both canals are surrounded by endothelium, 

 and the rod occupies the lower of the two. From this part forward, the 

 central canal is double, and the lower only has an endothelium of cylin- 

 drical cells, the upper being merely a space excavated in the parenchyma 

 of the spinal cord, . . . with only a slight lining of connective tissue. 

 At the posterior end of the medulla oblongata, the canal which is un- 

 provided with endothelium becomes very much enlarged, and a smaller 

 canal appears suddenly above it, which projects into the floor of the 

 fossa rhomboidalis and contains the central rod (Fig. 7). Tlie larger 

 canal, on reaching the base of the fissure on the dorsal surface between 

 the anterior end of the medulla oblongata and the posterior end of the 

 corpora bigemina, passes downward and forward in a curved direction, 

 and immediately beneath the posterior end of the posterior tuberosity 



