838 
ROSS GRANVILLE HARRISON 
low them little by little. Those that develop later, after the 
growth and shifting of the various parts of the organism has taken 
place, have much longer distances to grow, but the paths are 
already laid down by the pioneers and the later ones have only 
to follow where the others have led. 
Plexus formation by outgrowth is admirably illustrated in the 
nerves arising from the dorsal cells. These fibers begin their 
growth in a direction perpendicular to the axis of the body, and 
run intersegmentally approximately parallel to one another, 
being directed at first from the neural tube towards the epider- 
mis (figs. 1 and 3). Reaching the latter they begin to branch 
(fig. 6), though not perfectly regularly, owing to slight variations 
either in the nerve protoplasm itself or in the pathway. In branch- 
ing, a small deviation from the purely transverse direction of 
outgrowth takes place, and since the nerves run along the inner 
surface of the epidermis, and in fact are squeezed in between this 
layer and the muscle plates, the branches from adjacent segmental 
nerves must soon come together, and cross one another or form 
anastomoses. This last stage, which is not figured here from an 
actual specimen, may be readily observed in parasagittal sections 
taken just under the skin of Rana embryos about 6-7 mm. in 
length. It has been possible to observe the like of these proc- 
esses also in the live specimens in lymph; for instance, the forma- 
tion of branches, the crossing of two fibers growing in different 
directions, and the fusion or intimate contact (anastomosis) be- 
tween separate fibers that happen to come together. The dia- 
gram on p. 798 (text fig. 1) illustrates the process; the early stages 
are shown in figs. A and B, and the completion of the plexus in 
fig. C. This outline of development will account quite satis- 
factorily for the general features of a cutaneous plexus, i.e., defi- 
nite areas for each segmental nerve, a considerable overlapping 
of one segment of distribution upon adjacent ones, and minor 
irregularities in the mode of branching and anastomosis. A fur- 
ther feature, the oblique course of the nerve trunks in the lower 
part of the body and the tail, while perhaps in part due to the 
original direction of outgrowth, is largely brought about by the 
general shifting of the epidermis over underlying organs, which 
