148 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



A comparison of the diameter of Reissner's fibre in species of widely 

 difterent liabits leads one to the inference that there is a direct correla- 

 tion between the size of the fibre and the general activity of the animal. 

 In a general way one may say that in the group of teleosts the fibre 

 reaches its maximum size in the more active, swiftly moving types, of 

 which the bluefish (Pomatomus) is a good example, where the fibre has 

 a diameter of 10 micra. In the less active teleosts, other things being 

 equal, the fibre is smaller. In the sluggish and inactive goosefish 

 (Lophius), which is many times as heavy as the bluefish, the fibre has 

 a diameter of only one micron or less. 



In the light of present knowledge, after allowing for variability in 

 size of body, habits, etc., the size of Reissner's fibre seems to hold fairly 

 constant in the Ichthyopsida and Reptilia. In the trout and young- 

 alligator of appi'oximately the same weight the fibre is of the same 

 diameter, 3 micra. In the salamander (Necturus), which is somewhat 

 smaller, the fibre is 2.5 micra in diameter. In the higher groups, 

 birds and mammals, the fibre decreases in diameter. In the pigeon of 

 approximately the same weight as the salamander, the diameter of the 

 fibre is 1.8 micra. In the Mammalia the fibre seems to be still smaller, 

 though it has not yet been studied carefully enough in this group to 

 make generalizations safe. 



D. Morphology and Ontogeny of Reissner's Fibre and Its 



Cellular Connections. 



/. Cyclostomes. 



A. Historical. 



It was in the central canal of Petromyzon that Reissncr ('60) discov- 

 ered the fibre named for him, and the more important notices of it since 

 his time have been based on the study of it in cyclostomes. He knew 

 of its existence in the central canal only. It was not until twenty-two 

 years later that Mayser ('82) followed Reissner's fibre in teleosts into 

 the fourth ventricle. Sanders ('94) was the first to trace its course (in 

 Myxine) from the ventriculus terminalis anteriad into the mesocoele, 

 where he found it to divide into two branches. Studnicka ('99) fol- 

 lowed it similarly in Peti'omyzon, but failing to discover its termi- 

 nation anteriorly, concluded that it was non-nervous. Johnston (:02) 

 merely noticed its occurrence in the fourth ventricle of the brook 

 lamprey. 



