796 
ROSS GRANVILLE HARRISON 
cells which give rise to the dorsal nerves form a column in the 
dorso-lateral part of the wall of the medullary tube just within 
the external limiting membrane. In this stage certain of the cells 
are seen to have put forth fine branched processes, which extend 
for a short distance laterally in the notch between successive 
muscle plates (fig. 1, nf). The processes end in extremely fine 
filaments, so fine that their exact delimitation is often very diffi- 
cult to determine. The cell shown in the figure gives off another 
process quite as extensive as the one shown, but which is seen 
only in the section next to the one drawn. The structures in 
question are segmentally arranged, and correspond in the embryo 
under consideration to the intervals between the muscle plates 
from the second to the thirteenth segments. A much more 
advanced condition is shown in an embryo but very slightly 
older (3.7 mm. long). The dorsal nerves are here composed of 
several fibers in a bundle, each fiber connecting with a cell. The 
nerve shown in fig. 3 is composed of four such fibers (nf) which 
arise in pear-shaped cells (nbl) and converge toward the point 
where they lea\ e the medullary cord between the second and third 
myotomes. The endings are not shown in the section because the 
fibers bend just beneath the epidermis and run dorso-ventrally. 
They stain intensely with Congo red, as do the cone-shaped proc- 
esses of the cells from which they originate, and they show a fairly 
distinct fibrillation, even when stained merely by this method. 
The ends of the fibers are best seen in sagittal sections taken just 
between the epidermis and the underlying muscle plates. In a 
series of sections made from an embryo of Rana pipiens, 4 mm. 
long, they show particularly well. In fig. 4 the end of a bundle of 
three fibers situated between the ninth and tenth segments is 
shown. This terminal structure (npl) consists of a mass of hyaline 
protoplasm having a form suggestive of a rhizopod. The mass 
extends out into a number of very fine filaments. Such structures 
are found in each segment. Another, more highly magnified, is 
shown in fig. 5. Further towards the head of the embryo the 
fibers are longer and more branched (fig. 6), each branch ending 
in one of the peculiar enlargements just described. The young 
fibers of the r. ophthalmicus end similarly, although the ending 
