306 CARL CASKEY SPEIDEL 
OBSERVATIONS 
For purposes of description the fishes may be conveniently 
divided into the three groups indicated in the list above. In 
the first group cells homologous to the Dahlgren cells are lacking. 
In the second group the cells are present and are of moderate 
size, though varying somewhat. In the third group the cells 
are enormous and highly modified. 
In regard to the fishes of the first group, the alewife, eel, 
minnow, and toadfish, I know of no special reason why the cells 
of Dahlgren should be lacking. I have already suggested that 
they may be looked upon as having been derived from modified 
embryonic nerve tissues. A study of their development in skate 
embryos shows that they are derived from the same cells which 
give rise to nerve tissues. In these four fishes it would seem that 
there had never been any modification of the embryonic nerve 
tissues in the direction of the Dahlgren cells in this region, or so 
slight a modification that it is not easily noticeable. 
The second group varies somewhat with reference to the 
Dahlgren cells, both in size and appearance. In forms like the 
mackerel and goosefish the cells bear some resemblance, perhaps, 
to nerve cells. They are larger than nerve cells, but still rela- 
tively small. One cell of about average size in the mackerel 
measured 60u in length, 52 in breadth, and 39u in thickness. 
The cell processes are not numerous. The nucleus is usually 
only slightly lobulated, and may be almost spherical. One or 
more plasmosomes may be distinguished, the number often 
depending upon the number of lobes of the nucleus. In each 
of the cells of figures 1, 2, 3, and 4 taken from the mackerel 
one plasmosome is present (although in figure 2 it is not shown in 
the section represented). In the cell shown in figure 5 taken 
from the butterfish two plasmosomes are present. In one cell 
from the goosefish, the nucleus of which had three lobes, three 
plasmosomes were distinctly present, one in each lobe. Other 
forms, such as the catfish, cunner, garpike, menhaden, butter- 
fish (fig. 5), puffer, sea-robin, scad, scup, shark-sucker, tautog 
(fig. 6), tomeod, and whiting, show somewhat greater lobulation 
of the nucleus, plasmosomes sometimes being present, sometimes 
not. The cell-body is also larger in these forms. 
