6 Papers from the Department of Marine Biology. 
shape of the cells. Each cell usually has a few large, heavy processes 
which extend mainly in two directions. One of these is downward and 
slightly medial, while the other is lateral, the process or processes mark- 
ing a distinct boundary in transverse sections of the cord between an 
anterior and posterior portion of the white matter or fiber bundles of 
the region. In addition to these larger lateral processes, there were 
other and smaller processes which passed out anteriorly and posteriorly 
from the ends of the somewhat elongate cell. These smaller processes 
were not always observed. The position of these cells in cross-sections 
of the spinal cord is quite variable. Figure 1 shows two of them very 
close to the central canal. They may, however, be removed from it 
by some distance, as is the case in figure 21 (plate 5). 
The nucleus is of extraordinary structure, and aside from the large 
size of the cell is the most prominent feature to attract the attention 
when the cell is first examined (fig.5, plate 2). At first sightit appeared 
to be multiple in character and of marked irregularity, but when the 
series of sections of any particular cell was carefully examined, and 
when cells were reconstructed by wax plates or by graphic methods, it 
was seen that it was in reality but a single nucleus of the distributed 
type. Figure 8, plate 2, is a photograph of a wax reconstruction of the 
nucleus of a single cell, the same cell shown in cross-section in figure 5. 
There sometimes appears to be a center from which the principal 
masses of the nucleus branch, and this center is probably the original 
center. It is best seen in cells of Raia punctata. It is conspicuous for 
its lack of chromatic matter. This center is usually round in its gen- 
eral outline, like the nucleus of a muscle motor nerve-cell, and from it. 
the various branches of the nucleus are given off, beginning as narrow 
tube-like arms and branching in an irregular way as they pass toward 
the periphery. They get wider as they reach out from the center and 
at their terminal parts they enlarge into rounded portions, each of an 
equal or greater volume than the central part of origin. The extent 
of distribution varies, but its branches extend well towards the limits 
of the cell. Often the whole nucleus, expecially the central part, lies 
at one side and the branches embrace a central core of cytoplasm. 
Some nuclei have fewer and larger branches than others. 
The nucleus is separated from the cytoplasm by a very well-defined 
nuclear membrane, which is always continuous and intact. It has 
some thickness and staining capacity and its refractive index is higher 
than of most of the other cell-substances. 
The achromatic substance of the nucleus, the so-called nuclear sap, 
is about as thin or fluid in composition as in the usual nerve-cell. 
When fixed it is thrown into a reticular precipitate varying according 
to the fixative used, which serves to hold the numerous chromatic 
granules of the nucleus in their approximately true positions. This 
achromatic substance acts very much as mucin, not staining with iron 
