EARLY DEVELOPMENT OF MOTOR NERVE TRUNKS AND MYOTOMES. 123 
Heidenhain hematoxylin preparations that the young fibril appears to be made up of 
discrete particles arranged end to end.* But between these the protoplasmic matrix 
‘shows a more or less distinct fibrillar structure, the fibrils causing it having a longi- 
tudinal course and being continuous. These continuous fibrils appear to be an earlier 
stage of the contractile fibrils. ‘The outer wall of the myotome in transverse section 
has the appearance of being composed of a single layer of large cubical epithelial cells. 
In horizontal sections the cell boundaries are less easy to make out, but when visible 
they show that the cells form a truly cubical epithelium. During mitosis these cells 
become more or less spherical. 
Stage 31.—At this stage (text-fig. H, and figs. 12 and 12a) the outer portion of the 
muscle cell has increased in size, so that the outer limit of the contractile fibres is 
relatively much further removed from the outer end of the cell than it was before. In 
this outer end of the cell the cytoplasm assumes a clear transparent appearance, and in 
the» preserved specimens large clear vacuoles are seen which possibly in the fresh condi- 
tion contained glycogen. The inner part of the cell is now almost filled with contractile 
fibres, the protoplasm being reduced to the matrix between them. In this matrix yolk 
eranules are still abundant, and it is noteworthy that the muscle cells are now becoming 
multinucleate, the original nucleus having divided repeatedly. The division is mitotic. 
As the divisions only take place at relatively long intervals, a little patience is required 
in hunting for the mitotic figures. The resting nuclei lie free in the protoplasm of 
the myoblast. It is interesting, however, that during the period of mitotic activity the 
nucleus becomes surrounded by a sharply delimited more or less spherical mass of 
protoplasm, simulating the appearance of a cell within the myoblast. So striking is 
this appearance (PI. IV. fig. 14) that it suggested at first sight that more or fewer 
of the nuclei of the myoblast were really the nuclei of cells which had wandered into 
its substance from the mesenchyme without, just as such cells wander in later between 
the muscle fibres. On the whole, however, the balance of evidence is in favour of the 
cell-like structure round the nucleus being merely a temporary phenomenon due in 
some way to the influence of the mitotic activity of the nucleus on the surrounding 
eytoplasm—a phenomenon of the same nature as the rounding off into a spherical shape 
during mitosis of cells which in the resting condition are of more irregular outline. 
The appearance of the myotome of this stage, as shown in horizontal sections, is 
indicated in Pl. IV. fig. 12a. 
At about this period a striking change comes over the outer wall of the myotome. 
Numerous mitotic figures are observed in it. Its cells subdivide rapidly, so that the 
outer wall becomes several cells thick. The innermost of the cells so arising become 
squeezed in between the rounded ends of the primary muscle cells. At this stage it 
is often difficult to draw a line of demarcation between the outermost cells and those 
of the cutis which is now beginning to appear between myotome and skin. I am not, 
however, prepared to assert definitely that they actually give rise to cutis. 
* Cf. GopLEWSsKI, Arch. Mikr. Anuat., Bd. 1x., 1902. 
