308 CARL CASKEY SPEIDEL 
The third group includes only the flounder and the skate. 
The cells in these two forms are the most highly modified in size 
and differentiation yet found. The skate cells have already 
been described and a typical cell is shown in figure 9. This cell 
as seen in a cross-section of the spinal cord is elongated in two 
directions (ventro-medially and laterally), and at each of the 
ends is seen a number of granules, the secretion material of the 
cell. If the serial sections passing through this cell be followed, 
first in an anterior direction and then in a posterior direction, it 
will be found that the number of granules and volume of granular 
material both increase. The volume of granular material is 
greater at the anterior and posterior ends of the cell than at 
the middle region. The amount shown in the figure, therefore, 
represents only a small percentage of the total amount associated 
with the cell (much less than a tenth). The skate differs from 
all other forms that have been examined in the presence of large 
amounts of this granular material associated with the cells. 
Besides the skate, the flounder is perhaps the most interesting 
form. In this animal the spinal cord terminates posteriorly in 
an enlargement, the terminal enlargement, which extends an- 
teriorly for a short distance, forming a sort of cap over the dorsal 
part of the spinal cord. In this region, and for about an inch 
anteriorly, the spinal cord instead of appearing macroscopically 
as glistening white in color appears as a rather transparent gray. 
It is here that the Dahlgren cells are most numerous and of great- 
est size, the gray color of the spinal cord being due to their 
presence together with the lack of many myelinated nerve fibers. 
The cells are very large, about as large as in most species 
of skates. In the terminal portion of the spinal cord they are 
present in great numbers closely grouped together so that in both 
longitudinal and transverse sections they easily form the most 
conspicuous part of the section (figs. 12 and 13). A characteristic 
collection of the Dahlgren cells, as seen in longitudinal section, 
is shown in figure 12, a low-power microphotograph. It is 
interesting to note that in many of the cells in this section the 
nuclear material is located not at the center of the cell, but at 
the periphery. For this reason, the cell looks somewhat like a 
