CLARKE, ON STRIPED MUSCULAR FIBRE. 3 
process of formation. Fig. 21 ¢ represents different kinds of 
these fibres. In the bundles which they form they lie in 
such close apposition that they appear to be almost cemented 
together. One of these is represented at fig. 2le. At its 
lower part the fibres have become separated. At the sides 
of such a bundle it was not uncommon to find oval nuclei 
with processes which divide into branches, as shown in the 
figure. Sometimes several nuclei appeared to be joined to- 
gether by a condensation of the intervening blastema, in 
which at the same time a kind of plexus of fibres, of very 
small but variable diameter, became developed (fig. 21g). In 
the heart the fibrille were much more frequently resolved 
into particles or sarcous elements, and therefore the appear- 
ances of transverse strize were much more common than in 
the trunk. 
In foetuses of one and a half or two inches in length the mus- 
cular fibres of the trunk, which were first developed, had in- 
creased considerably in diameter ; but many smaller ones were 
either formed or in process of formation. Fig. 22 represents 
several fibres in different states of development, from an arm 
of a human feetus about two inches in length. Their increase 
in diameter depends, in some places, partly on a certain in- 
crease in the size of the nuclei which they contain, but chiefly 
on the deposition of new layers of the substance or the fibrillz 
by which they are invested, and which, therefore, extend the 
breadth of the original borders. In the majority of instances 
these new layers are deposited nearly equally round the axis, 
but in many others they are added—at least for a variable 
length—more thickly on one side, as shown at a, fig. 22; so 
that from this cause, as well as from the size and relative 
distance from each other of the nuclei, the same fibre may 
vary in diameter at different parts of its course. It is flatter 
also in some parts, and gradually assumes a more cylindrical 
shape and uniform structure throughout its entire thickness. 
Numerous nuclei lie on its surface, along which granular 
processes may be frequently seen to extend from one to the 
other, as the foundation of new fibrillz (see fig. 226). In all 
the larger fibres, and in most of those of intermediate size, 
the striz are beautifully marked, but have often a different 
aspect in different fibres and in different parts of the same 
fibre. On each side of the axis there is commonly observed 
a very remarkable border of transverse striz, corresponding 
to the plain lateral borders, and indicating the depth of the 
fibrillation. Fig. 22y is an exact representation of a large 
and strongly marked fibre from the same fcetus. Its striated 
border, under a sufficient magnifying power, was easily re- 
solved into several rows of sarcous particles, like those re- 
