OUTGROWTH OF THE NERVE FIBER 
795 
sidered, an embryo of R. sylvatica, 4.1 mm. long, the beginnings 
of the peripheral nerves, and of some of the principal central 
bundles are plainly visible. Of all the peripheral nerves the 
r. ophthalmicus of the trigeminal, seems to be furthest advanced. 
A very early phase of this nerve is shown in fig. 2 (n/), drawn from 
an embryo of R. esculent a, 3 mm. long, which is in about the same 
stage of development as the sylvatica embryo just mentioned. 
Protoplasmic processes of the cells within the ganglion are seen 
to extend for a short distance into the mesenchyme, without hav- 
ing any special relation to the cells of that tissue. The ends of 
the processes are branched and filamentous. In the sylvatica 
embryo under consideration, a considerable number of peripheral 
nerves in addition to the ophthalmic are alread^^laid down. There 
are at least four ventral spinal roots, corresponding to the sec- 
ond, third, fourth and fifth muscle plates, to which they may be 
traced; several of the dorsal nerves of Rohon-Beard, extending 
out between the myotomes and the epidermis; and some fibers in 
the r. lateralis vagi J 
The early characteristics of the developing nerve are most 
clearly shown by the fibers which originate in the dorsal cells of 
Rohon-Beard. These grow just beneath the epidermis iu the 
space between the muscle plates, where at this period there are 
no loose mesenchyme cells, and they remain free from sheath 
cells throughout their growth. The clearest cases of the earliest 
beginning of these nerves have been found in an embryo of Rana 
palustris, 3.6 mm. long, which is almost identical in degree of 
development with the sylvatica embryo just described. The 
son of the protoplasmic bridges fixed in osmic acid with those seen after fixation 
in the usual preservatives made upon such vertebrate embryos as those of the 
selachian, the teleost, or the bird, in which there is little or no yolk in the tissues 
at the time when the first nerve fibers differentiate. 
^ These early nervous connections, which are important for the proper inter- 
pretation of the relation between structure and function in the neuro-muscular 
system, have been ignored by a number of investigators. In his histogenetic 
study of the nervous system O. Schultze ('05) has overlooked these stages of de- 
velopment completely and has thereby been entirely misled in his views regarding 
the early development of nerve fibers and the formation of the cutaneous plexuses 
(Cf. Harrison -'04, '06). Held ('09) has recently subjected Schultze's work to a 
searching criticism, all the main points of which seem to be entirely justified. 
