122 PROFESSOR J. GRAHAM KERR ON SOME POINTS IN THE 
This is at first very thick and conspicuous, but it gradually thins out, its yolk is con- 
sumed, and eventually the only parts remaining conspicuously visible are the nuclei 
dotted along the surface of the nerve trunk. 
Into the subsequent history of the motor nerve—which is of minor morphological 
interest—I do not propose at this time to enter in detail. 
The protoplasmic sheath grows into the nerve trunk, dividing it up into separate 
bundles of fibrils. The nerve trunk, as has been shown, spreads out at an early stage 
in conical fashion over the inner face of the myotome. As the myotome grows in 
surface this cone becomes broken up into distinct strands which become more and 
more divergent. 
As the adult condition is reached the part of the nerve trunk proximal to the point 
of divergence—iz.e. to the apex of the cone—increases relatively little in length. The 
distal portion, on the other hand, with its dividual branches, increases enormously in 
length and the branches become more and more changed in direction as the muscles to 
which they are attached become pushed about by differential growth. 
DEVELOPMENT OF THE MYOMERES. 
The general features in the development of a typical myomere as seen in transverse 
sections are shown in text-figures A to H,* and in detail in PI. III. and IV. figs. 11-14. 
The protovertebra, at first (text-fig. A) a solid diverticulum of the enteric rudiment, 
develops a myoccelic cavity through the breaking down of its central cells about 
stage 21 (text-fig. B). 
By stage 24 (text-fig. C) the myotome is beginning to show signs of a flattening 
from without inwards. The myocele is obliterated, and the cells of its mesial wall have 
become flattened in form. At their outer ends they interdigitate with the inner ends of 
the cells of the outer wall, so that the line separating the two walls in a transverse 
section is a zigzag one. 
In stage 27 (text-fig. D, Pl. III. fig. 11) the inner wall cells have become more 
regular in shape; forming rectangular parallelepipeds flattened in an obliquely dorso- 
ventral direction, so that their larger faces slope inwards and downwards. Contractile 
fibres (fig. 11, c.f.) have now appeared in the body of these cells, running longitudinally 
and forming most frequently a layer close to each of the dorsal and ventral surfaces, the 
two layers becoming frequently continuous with one another externally, and sometimes 
internally. Very often, however, the arrangement of fibres at their first appearance 
does not show this regular arrangement. ‘The contractile fibres appear to be rounded in 
section, and are easily distinguished by their highly refringent character, and by their 
peculiarly deep stain with Heidenhain’s hematoxylin. The cells of the myotome are at 
this stage still laden with yolk, and this naturally is a difficulty in the way of observation. 
The muscle fibrille are striped almost from the beginning. One can often see in 
* These have been printed as separate Plates—V. and VI. 
