SARGENT. — REISSNER'S FIBRE. 447 



teroiinalis. The surrounding walls of the canal are made up of a single 

 layer of cells, actively dividing. Certain of these cells become separated 

 from the walls and pushed into the lumen. Of these many atrophy 

 and disintegrate. Some eight to twelve of them persist and continue to 

 develop. These cells, at first spherical, become spindle-shaped, as they in- 

 crease in size. These posterior canal cells (as I temporarily call them) 

 give off peripherally a number of dendrites which penetrate the tissue 

 of the cord (Figure 3). From the taperiug anterior end of the cell is 

 given off the axon (Figure 4), which runs cephalad through the lumen, 

 eveutuaJly coalescing with the axon of similar cells (Figure 5) and 

 then continuing its growth cephalad. About the third day this system 

 of forward-growing axons meets the system of axons from the cells of 

 the tectum growing backward and the two systems coalesce in a way 

 not yet clearly made out, to complete what has been called Keissner's 

 fibre. This fibre, then, is a nerve tract, composed of axons running in 

 opposite directions, both cephalad and caudad. The development of this 

 apparatus as outlined for Amia is typical of all vertebrates. 



The condition of the apparatus in Selachii is especially interesting. 

 In the skate Raja erinacea the cells are of great size (25 fj. to 30 /x in 

 diameter), being the most conspicuous elements in the brain. They are 

 three hundred to four hundred in number, and lie in two bands, one on 

 either side of the median plane, extending the whole length of the roof 

 of the optic lobe (PI. 2, Figure 9). At the anterior end these two bands 

 meet, forming a large group of closely aggregated cells. The cells are 

 multipolar, giving off" several processes in addition to the large axon, which 

 is 2 /A to 3 ^u. in diameter. As in Amia, the nucleus is eccentric, being 

 always at the side of the cell opposite that from which the axon emerges. 

 The number of the capillaries in the neighborhood of these cells, supply- 

 ing them with blood, gives evidence as to the extent of the activity of the 

 cells. 



Although these cells are so conspicuous in the brain of the skate, they 

 have attracted but little attention. They were first seen by Rohon in 

 selachians and correspond in position to the " Dachkerne " which have 

 been described for Amphibians and Reptiles, the function of which has 

 not been established. 



The axons pass dorsad and laterad from their cells, turning either 

 cephalad or caudad, and form two great tracts of non-medullated fibres of 

 large size, which extend through the whole tectum opticum lateral and 

 dorsal to the cells. The fibres running caudad can easily be traced to 

 the posterior end of the tectum, whence they pass into the cerebellum. 



