Mr. J. L. Clarke on the Development of Muscular Fibre. 237 
As incubation advances, the fibres acquire a tubular investment of 
the contractile or sarcous substance, which gradually increases in 
thickness or depth, and appears on each side as a band of corre- 
sponding breadth. As they grow in length, they also contract in 
diameter, and become of uniform structure throughout ; while their 
nuclei rise nearer to the surface, and assume a more oval form. At 
this period the marks of striation, either longitudinal or transverse, 
are only faint and occasional. 
By the fourteenth day of incubation, the entire substance of the 
fibres separates into longitudinal fibrillee, which in turn become shortl 
resolved into particles or sarcous elements. After this the fibres 
continue to grow in thickness by the addition, to their surfaces, of 
new fibrillee, which, as usual, are formed around nuclei encrusted 
with blastema cementing them, in such cases, to the original fibre. 
In mammalia, although there are some particular but unimportant 
differences in the development of muscular fibre, the general plan is 
the same as in birds. The nuclei—at least in the ox, sheep, and 
pig—are larger, and have more distinct cell-walls or enveloping 
membranes. The fibres of the sheep or pig first make their appear- 
ance, in the foetus of from half to three-quarters of an inch in length, 
as thick and nearly parallel threads lying amongst a. densely crowded 
mass of free nuclei. When isolated, these fibres are seen to be attached 
to one or more of the nuclei by a variable quantity of blastema. 
Sometimes a single nucleus with conical processes of delicate granular 
substance is first enclosed by fine fibrillee or lateral bands, which 
present somewhat the appearance of a cell-wall, so that the object 
has a certain resemblance to a nucleated fusiform cell with a fibre 
originating from one of its extremities. Sometimes several nuclei are 
cemented in a group around a fibre, and become subsequently covered 
by other fibres of the same kind; and sometimes they lie in linear 
series, either at some distance apart, or overlying each other toa cer- 
tain extent like a series of coins. The lateral bands or fibres enclosing 
the nuclei extend around them as a tubular investment, which grows 
in thickness from without, but not always uniformly on all sides. 
In the process of longitudinal growth, the nuclei multiply by subdi- 
vision, become generally more oval, and approach nearer to the sur- 
face of the fibre, which at the same time contracts in diameter. The 
subsequent changes they pass through are nearly similar to those 
which occur in the chick. 
In man the development of muscular fibre proceeds on the same 
general plan as in birds and mammalia, but differs from that of both 
in certain unimportant particulars. In the early stages there is.no 
distinct appearance of those oval, cylindrical, and irregular masses 
observable in the chick on the seventh day of incubation and in the 
mammal at a corresponding period. In this respect there is a greater 
resemblance between the two latter classes than between man and 
either. In the human fcetus, from about half to three-quarters of an 
inch in length, the first stage of development may be seen to com- 
mence by the formation of fine lateral bands or fibrillee along one or 
both sides of one nucleus or more. When, however, there are more 
nuclei than one enclosed by the same lateral bands, they are always 
Ann. & Mag. N. Hist. Ser. 3. Vol. ix. iz 
