124 PROFESSOR J. GRAHAM KERR ON SOME POINTS IN THE 
The outer wall cells now become converted into elongated irregularly cylindrical 
cells which stretch continuously from one muscle septum to the next. In the proto- 
plasm of these there begin to appear about stage 31+ (Pl. IV. figs. 13 and 134, and 
text-fic. F) contractile fibrils of a similar nature to those which have been long present 
in the inner wall cells. About stage 31 mesenchyme cells wander in between the 
myotomes. These give rise to the substance of the septum. Some also wander in 
between the muscle cells of the outer wall. 
By stage 34 the layer of muscle cylinders arising from the outer wall much exceeds 
the inner wall in thickness. It is distinguished from the latter at the first glance, its 
muscle cells being slender cylinders instead of flattened parallelepipeds. 
The general appearance in transverse section of a myotome of this stage will be 
gathered from text-fig. G. It will be seen that the lateral branch of the vagus nerve 
with its surrounding mesenchyme has formed an immovable obstacle, so that the 
myotome as it increases in thickness, and as it is pushed outwards by the increase of 
mesenchyme on its inner side, becomes gradually divided into two portions, a dorsal 
and a ventral, the two remaining for a time connected by a thin isthmus but being 
eventually completely separated. Along the mesial face of the myotome are seen the 
muscle cells of the inner wall, now reduced greatly in size in proportion to the whole 
thickness of the myotome. 
The whole thickness of the myotome outside this consists of the derivatives of the 
outer wall. In its extreme outer portion it consists of cylindrical cells still in the 
condition described for stage 31, in which contractile fibres are just beginning to 
appear. rom this in a mesial direction the muscle cells are seen to become of greater 
and greater diameter, and their contained contractile substance increases, especially in 
size, Showing that the muscle cells are older as they are farther removed from the outer 
surface. 
This, together with the fact that mitotic figures are numerous in the external 
layer, shows that this latter is the region in which growth in thickness of the myotome 
takes place. 
The myoepithelial cells of the inner wall remain distinct up to about stage 35, 
though constantly becoming more and more insignificant as compared with the great 
mass of the myotome composed of muscle cylinders derived from the outer wall. 
About the stage mentioned, however, the myoepithelial cells begin to break down, 
portions becoming segmented off from their outer ends. These resemble the muscle 
cells of the outer layer in character, being long cylinders. This process goes on, and 
soon the once conspicuous myoepithelial cells have become entirely resolved into these 
cylindrical elements, and the myotome is composed of apparently similar elements 
throughout its thickness (text-fic. H). 
