208 bulletin: museum of comparative zoology. 



of the cells near the ventral surface of the torus pass directly into the 

 mesocoele along the whole ventral surface of the torus, but more 

 numerously near the median plane. In the ventricle these coalesce and 

 run together in the median plane forming a diffuse dendritic system of 

 trunks and branches (Fig. 58, fas. Reis.). In older specimens the 

 number of branches is greatly diminished. The cells from the dorsal 

 and anterior parts of the torus send their axons into the ventricle 

 cephalad of the posterior commissure. At an early stage of develop- 

 ment these axons emerge from thevvhole upper surface of the ventricle 

 singly or in numerous small trunks, which are directed caudad and 

 toward the median plane. These unite into a single trunk which, 

 caudad of the posterior commissure, unites with the other system to 

 form Ileissner's fibre. In the adult the whole mass of axons in front 

 of the posterior commissure emerges into the ventricle as a few con- 

 solidated trunks. 



At its posteiuor termination the canalis centralis expands into an 

 ovoid, almost sphei'ical, ventriculus terminalis. In the canal beyond the 

 heterocercal flexure I have found a number of small posterior canal- 

 cells. 



In the nearly related families examined no important observations 

 have been made. In the species Siphostoma fuscum, Mugil cephalus, 

 Ammodytes americanus. Scomber scombrus, and Sarda sarda, Reissner's 

 fibre is of small size, but has been found and traced for greater or less 

 distances through the canal and brain ventricles. 



In the rudder fish, Naucrates ductor, Reissner's fibre has a diameter 

 of 2.5 micra. 



PoMATOMiDAE (126). Reissner's fibre in the bluefish, Pomatomus sal- 

 tatrix, is of large size, having in the adult a diameter of-8 to 9 micra in 

 its course through the anterior portion of the canal and brain ventricles. 

 In the mesocoele below the valvula cerebelli, it is divided into two or 

 three main trunks {fbr. Reis. Fig. /). One passes above the posterior 

 commissure (trt. tor. fbr. Reis. p.), and between it and the valvula 

 cerebelli into the fissure between the lobes of the torus, and there again 

 divides into two lateral branches, whicli enter the lobes of the torus, 

 "where they break up iuto fibre-tracts which have their origin in the cells 

 of the torus. The other main trunk continues cephalad, ventral to the 

 posterior commissure and below the ' Schaltstiick,' and divides several 

 times (irt tor.flrr. Reis. a.). The divisions enter between the ependymal 

 cells and thence run dorsad and laterad into the torus longitudinalis, 

 ^vhere they break up into their constituent axons. One or two fine 



