160 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



places a curiou.slj twisted appearance. lu some speciraeus the chamber 

 above mentioned, at the posterior end of the spinal cord, is occupied by 

 a mulberry-shaped mass of glass-like aspect, from which, as from a 

 knotted end, the rod in question emerges, in other specimens the rod is 

 attached to the surrounding connective tissue." Studnicka ('99, p. 9) 

 has also described and figured this "Endknauel" in the ventriculus 

 terminalis of Petromyzon and Myxine. He describes the portion of 

 the fibre forming the knot as of indefinite outline and varicose form, 

 and accounts for its condition in these words : " Der Knoten auf dem 

 caudalen Ende des Reissner'schen Fadens entsteht jedenfalls nicht 

 allein in loco, in dem Ventriculus terminalis, sondern der Faden schiebt 

 sich aus dem Canalis centralis (bei den Bewegungen des Thieres?) all- 

 mahlig in den Ventrikel hinein, und biklet, da er hier wenig Platz 

 fiudet den Knauel, und wird endlich in der Grundsubstanz des lockeren, 

 von Lyniphraiumeu stark durchgesetzten Schleimgewebes aufgelost." 



The explanation of this tangled and shrunken condition of Reissner's 

 fibre, as I have seen it in the ventriculus terminalis, is not difficult. 

 Observations made on the freshly isolated Reissner's fibre of sharks 

 show that it is elastic, and tends to coil and shrink under the action of 

 fixing fluids (pp. 144, 174). It is therefore entirely reasonable to suppose 

 that when, in the fresh condition, the cord was severed, Reissner's fibre 

 recoiled an'd shrank back into the ventricle, the posterior end being the 

 one at which it was most firmly held. The shrinking and coiling was 

 perhaps augmented by the action of the fixing fluid. A similar coiling 

 of the cut end of the fibre within the fourth ventricle in teleosts is 

 described elsewhere in this paper (p. 211). 



The increased thickness and loss of definite outline may be accounted 

 for in the same way. No such coiling and shrinking has been ob- 

 served in any case where the animal has been fixed before severing the 

 fibre. Studnicka's statement that the end of Reissner's fibre passes 

 out of the sinus of the ventriculus terminalis and into the surrounding 

 lymph space is so at variance with all my observations that I must 

 believe the appearance he so interprets was accidental and due to the 

 disturbed and abnormal condition of the fibre in his prepai'ations. 



D. Summary for Ctclostomes. 



In cyclostomes the optic reflex apparatus is in many respects in a 

 primitive condition, but the relative size of its elements shows it to be 

 of great importance in tlie activities of the animal. It is relatively late 

 in development, not being fully established until the second month of 



