EARLY DEVELOPMENT OF MOTOR NERVE TRUNKS AND MYOTOMES. 125 
GENERAL REMARKS. 
1. Motor Nerve Trunks. 
As is well known, there are three main views regarding the development of motor 
nerves in the vertebrata, which may be shortly stated as follows :— — 
(1) Each nerve fibre develops as an independent outgrowth from a ganglion cell 
which gradually grows out towards, and finally and secondarily becomes united to, its 
special muscle. The sheath of protoplasm surrounding the nerve is an accessory 
structure of independent origin developed from mesenchyme. This view is associated 
especially with the name of His, and is the view favoured by the majority of 
embryologists. 
(2) The nerve trunk is multicellular in origin, consisting at first of a chain of cells, 
in the substance of which the nerve fibres are developed later, as fine fibres passing 
continuously from one cell body to another. The elements forming the original chain 
are most frequently looked on as ectodermal elements which have wandered out from 
the spinal cord rudiments. The protoplasmic sheath is derived from parts of the 
original cell chains which retain their protoplasmic character (BaLFour, GOETTE, 
Bearp, Dourn, v. WisHE, and others). 
(3) The nerve trunk is not a secondarily formed bridge between spinal cord and 
motor end organ. It has existed from the first, and in subsequent development it 
merely undergoes elaboration from its at first simple protoplasmic beginning (v. Bazr, 
Hensen, SEDGWIick, FURBRINGER and others). 
It is clear that the facts of development in Lepidoszren, at least in the motor nerves 
of that animal, give strong support to the last-mentioned view as regards the nerve 
trunk itself, and to the second view as regards the protoplasmic sheath. It has been 
shown that by the examination of earlier and earlier stages the motor trunk can be 
traced back, without, I think, any possibility of error, to a simple protoplasmic bridge 
which already connects the substance of the medullary tube with that of the myotome 
at a stage when they are still in contact. 
As regards the origin of the protoplasmic sheath the evidence of Lepidosiren is 
equally emphatic. In its early stages the motor trunk is perfectly naked. About 
stage 27 masses of mesenchymatous protoplasm Jaden with yolk become applied to the 
nerve trunk, at first over only a small portion of its length, and these masses of 
protoplasm gradually spread over the whole trunk, remaining, however, for some time 
clearly distinguishable from the nerve trunk by their difference in staining reaction. 
As development goes on the yolk becomes used up, the protoplasm with its nuclei 
extends into the substance of the nerve trunk—doubtless to keep up the proper 
proportion between the bulk of the nerve trunk and its nutritive surface in contact with 
the sheath protoplasm. The protoplasm itself becomes less and less conspicuous, and 
eventually is only to be detected in the immediate vicinity of the nuclei. 
