186 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



process of division (cZ. r/., Plate 4, Fig. 27, Plate 5, Fig. 39). Certain of 

 these cells become separated from the wall of the canal and pushed into 

 the lumen. -^ I have seen such cells still an integral part of the wall but 

 projecting into the canal. In Figure 30, Plate 4, two cells (a, a) are shown 

 lying close to the wall, from which they have been given off and with 

 which they are still connected by protoplasmic strands. Tiiese two cells 

 are in contact with each other, and are apparently connected with a 

 third (y) by a protoplasmic bridge. In the same figure the two small 

 cells (^) in the same region are perhajis to be thrown off later, or they 

 may represent the residual halves of the mother cells from whicli a, a wore 

 formed. Such cells usually continue for a considerable period to lie 

 close to the portion of the wall from wliicli they were given off, and 

 probably always maintain their connection with it (Plate 4, Fig. 29). 

 In Figure 27 are shown two small cells (a) lying at the side of the vcn- 

 triculus terminalis, close to one of the actively dividing germinal cells, 

 'Keimzellen' of His ('89), from which they may have been derived. In 

 the upper portion of the figure are several dorsal cells (' Hinterzellen ') 

 in an early stage of development, some of which are being pushed out 

 from the cord. Of these cells in the canal and terminal ventricle, many 

 atrophy and disintegrate. Some eight to twelve persist and continue to 

 develop. These, at first spherical in form, become somewhat spindle- 

 shaped as they increase in size (Plate 5, Fig. 39, d. can. 2>-)- 



The axon is given off from the more tapering anterior end of the cell 

 and runs forward through the lumen of the canal. The developing 

 axon is exceedingly delicate, and cannot be followed to its tip, as it 

 becomes finer and finer till it fades away (Plate 4, Fig. 31, a). From 

 what I have seen I believe the process of growth of the axon, as in the 

 cells of the tectum, must be similar to the streaming movement of the 

 pseudopodia in certain Heliozoa, and not at all like the growth of a 

 root-tip. At first the axons of these cells are quite separate, but 

 eventually they coalesce in much the same manner as do the axons of 

 the tectal reflex cells. Two or more axons may unite to form a single 

 fibril, which in turn uniting with similar fibrils forms the main trunk 

 running cephalad (Fig. 31). These separate fibrils may run parallel for 

 some distance before uniting. This I found especially true in the cat- 

 fish (Plate 8, Fig. 61, ax.'), where as many as four such fibrils may be 

 found in one cross-section. This perhaps offers an explanation of Stud- 



1 A similar process takes place normally in the central canal during larval 

 life at least. Cells from the walls of the canal are pushed out into the lumen, 

 where they may be found in various stages of disintegration (see footnote, p. 143). 



