SAKGENT : THE OPTIC KEFLEX APPAKATUS OF YEKTEBKATES. 107 



of the brain, enlarges into a quadrangular or rounded chamber (Fig. 2), 

 the whole looking like a pipe-bowl and its stem. The central rod 

 comes forward from that part of the canalis centralis which is situated iu 

 the spinal cord, and passing into the enlarged canal in the medulla 

 oblongata, there divides in two parts, one part goes straight forward 

 in the small upper canal before mentioned on the floor of the sinus 

 rhomboidalis, the other passes through the enlarged canal into the 

 quadrangular chamber, and there forming a knot, goes upward, and is 

 lost in the aqueduct of Sylvius." 



Studnicka ('99, p. 7) thus describes the course of the fibre: "Bei 

 Myxiue dringt der Faden, nachdem er den engen, die Stelle des Ventric- 

 ulus IV hier vertretenden Canal durchgelaufen hat, ebenfalls in die 

 Masse des Kleinhirns und endigt in der Hohle des Mittelhirns. Der 

 ganz enge Canal, in dem der Faden die Kleinhirnmasse durchtintt, ist 

 nicht besonders ausgekleidet." Neither of the above investigators seems 

 to have found the fibre embedded in the lobus acusticum, as I have 

 described it. 



The passage of Reissner's fibre fur a part of its course through the 

 nervous tissue in cyclostomes is especially interesting in throwing light 

 on a structure in Amphioxus little understood. Of the dorsal giant 

 nerve cells in the anterior portion of the central nervous system of 

 Amphioxus, the largest and most anterior (designated by Rohde ('88) as 

 the 'Kolossale Ganglienzelle, A') is median, and lies across the lumen 

 of the central canal. It is multipolar, sending, according to Retzius 

 ('90), several small processes to the pigment spot at the anterior end 

 of the brain, which is generally regarded as a primitive eye. Its large 

 axis-cylinder runs caudad in the median plane, just ventral to the canal. 

 I believe it is probably connected posteriorly with the musculature, and 

 is motor in function. 



The direct connection of the cell with the eye, the location of the 

 cell dorsally at the extreme anterior end of the nervous system, and 

 the jiassage of the large unmedullated axis-cylinder caudad in the 

 median plane, all suggest strongly a similarity to the optic reflex appara- 

 tus of cyclostomes. I venture the conjecture that the giant axis- 

 cylinder of this cell in Amphioxus represents in a primitive condition 

 Reissner's fibre of the Craniota. In Amphioxus, where the central canal 

 is not sharply defined and the central nervous system is loosely organ- 

 ized, Reissner's fibre runs its whole course through the nerve tissue. In 

 cyclostomes its course is, for the most part, through the spacious brain 

 ventricles and canal, but for a short portion of its course it still passes 



