10 Papers from the Department of Marine Biology. 
HISTOGENESIS. 
A few skate embryos of the species Raia punctata were collected and 
sectioned by Dahlgren at the Zoological Station at Naples. This 
material was turned over to the writer. The embryos were in a suffi- 
cient number of stages for the origin and development of the large, 
remarkable cells in question to be worked out. Stages were traced 
that showed them being differentiated from the same cells that also 
give rise to the motor nerve-cells of the anterior horn. 
A typical series of early developmental stages was found in an em- 
bryo 7.3 cm. in length. The yolk-sac of this embryo was still fairly 
large. Sections taken at the level of the first tail-fin show how these 
cells become differentiated from the ordinary type of nerve-cell (figs. 9 
to 16). There is first an increase in the amount and number of chro- 
matin granules. The nucleus loses its spherical shape and becomes 
elongated (figs. 10, 11, 12). At a slightly later stage the nucleus be- 
comes lobular as a result of unequal and localized growth (figs. 13, 14). 
The chromatin granules become more numerous. The first six figures 
of this plate have been drawn from cells from a single skate embryo. 
It is evident that all of the cells are not in the same stage of develop- 
ment. Some have hardly begun to show any differentiation at all, 
while others close to them have become well started on the growth and 
development that leads to the peculiar cells of the adult. Figure 17 is 
a drawing of a well-differentiated nerve-cell taken from the same 
embryo. It will be seen that this nerve-cell has already developed 
most of the typical nerve-cell structures, so that it is much like the 
nerve-cells of the adult. However, other nerve-cells are present 
which are less far along in their development. 
Figures 15 and 16 are cells from the spinal cord of an embryo 22 cm. 
in length. The nucleus has become much larger and more lobular and 
the chromatin granules have also multiplied. The later development 
of the nucleus is merely a continuation of the process of unequal and 
localized growth, until finally it assumes the highly irregular, branching 
form that it has in the adult skate. The size of the cell-body as well as 
the size of the nucleus increases enormously, so that in sections of the 
spinal cord taken well down toward the tip of the tail this cell is one of 
the most conspicuous things in the field. 
COMPARATIVE STUDY. 
Several other kinds of fishes have been examined to see whether or 
not these cells are peculiar to the skate only. Cells resembling them 
more or less closely in appearance have been found in the dog-fish, 
shark, sting-ray, torpedo, goose-fish, gar-pike, and sword-fish; not, 
however, in Fundulus. It is not intended to give here a complete 
description of the morphology of these cells in each of the animals men- 
