120 PROFESSOR J. GRAHAM KERR ON SOME POINTS IN THE 
Appearances, in fact, support the Kettentheorie—suggesting a chain of “cells” placed end 
to end. More careful examination shows the presence within the protoplasmic strand 
of a cord, faintly fibrillated longitudinally, and differentiated from the simple proto- 
plasm by its affinity for eosin. The nerve runs downwards along the inner face of the 
myotome, and it is difhcult to make out with certainty its connections with the cells of 
the myotome. 
Stage 29.—At this stage (figs. 2 and 7) the conducting part of the nerve (7.t.) is in 
a similar condition to that described for stage 30—a distinctly fibrillated band— 
usually about 2°5 to 3u in thickness close to its root—which stains deeply with eosin. 
It slopes outwards and downwards from the ventrolateral angle of the spinal cord to 
the inner surface of the myotome, along which it proceeds in a ventral direction. The 
protoplasmic sheath (p.s.), however, is now far more conspicuous than in later stages. 
It is a great, irregularly thickened mass of granular protoplasm, sharply marked off from 
the true nerve by its being stained more greyish in colour in hematoxylin and eosin 
preparations, in sharp contrast to the deep red of the nerve trunk. 
Scattered through the protoplasm of the sheath are large nuclei rich in chromatin, 
yolk granules, and here and there vocuoles. 
Stage 27.—At this stage (figs. 3 and 8) the most conspicuous difference from stage 
29 is that the nerve trunk (v.?.) is now naked for the greater part of its length. At its 
outer end it spreads out into a number of strands arranged in a conical fashion. In the 
case of the strands near the axis of the cone—z.e. in the case of the strands which 
pursue a direct course towards the inner surface of the myotome—it may be clearly seen 
under a high power of the microscope that each strand passes perfectly continuously and 
by insensible gradations into the granular protoplasm, which forms a tail-like process of 
a myoepithelial cell of the myotome. In the case of many of the motor trunks at this 
stage there is to be seen a mass of mesenchymatous protoplasm (.s.) richly laden with 
yolk, and containing numerous nuclei, concentrated in the neighbourhood of the nerve 
towards its outer end. This is the rudiment of the mesenchymatous sheath which in 
stage 29 we saw had spread out over the surface of the nerve. The nerve trunk 
itself is about the same thickness as in stage 29, though I find considerable variation 
in this respect. 
Stage 25 (figs. 4 and 9).—A little behind the middle of the body at this stage the 
myotome is seen to be just commencing its recession from the spinal cord, mesenchyme 
(me.) cells* richly laden with yolk having begun to migrate in between the two 
structures. The nerve rudiment (n.t.) in the section figured is about 7 thick. It is” 
distinctly fibrillated, and at its lower end expands into a cone as in stage 27—the base 
of the cone, however, here being less expanded. Traces of longitudinal fibres are already 
visible on the ventrolateral surface of the spinal cord. 
* Here as elsewhere I use the word “cell” merely as a substitute for the more cumbrous expression “ nucleated 
mass of protoplasm” without in the least implying that it is separate from its neighbours. As a matter of fact the 
“cells” of the mesenchyme are merely the enlarged and nucleated nodes of an irregular continuous protoplasmic 
spongework such as Spp@wickK describes in Selachians. 
