STRIATED MUSCULAR FIBRE IN MAMMALIA. 
249 
lengthen and decrease in diameter. The only difference is, that owing to the later 
period at which this occurs, the majority of the nuclei perish previously, and the 
distinction between the lateral bands and the central portion of the fibre grows 
obscure ; hence the process is not so obvious, because the separation of the nuclei, 
when they remain, is a most remarkable feature. But the extension of the clusters 
of granules longitudinally, the wide separation of the granules from one another, 
and the remarkable decrease in the diameter of the fibre, are not to be mistaken. 
(See Plates X. & XI. fig. Q d, e,f8L g.) 
It is to be observed, that the longer the lengthening of the fibres is delayed, the 
thicker do the lateral bands become. The addition of fresh material from without, 
which, when the fibre is elongating, is consumed in maintaining the bands at their 
original width, now adds very much to their breadth, so that these fibres often attain 
a very considerable diameter. On the contrary, in those fibres in which the length- 
ening occurs at an earlier period and is more rapidly accomplished, tlie lateral bands 
being as yet very thin, the fibre thus formed is very narrow. Indeed, they are some- 
times narrower in the intervals between the nuclei than are the nuclei themselves ; 
so that, where these are situated, the fibre is necessarily bulged to contain them. 
This appearance may, however, be caused by stretching the fibre while preparing it 
for the microscope. 
The striae first become visible at this period. A faint indication of their appear- 
ance may be sometimes observed in the lateral bands, almost as soon as these are 
fully formed ; and as these bands approximate, the striae become more plainly marked, 
and often contrast strongly with the intermediate and apparently homogeneous cen- 
tral portion of the fibre. In any case they can always be readily detected at the time 
when the distinction between the lateral bands and axis disappears. 
The striae are first discerned immediately within the margin of the fibre, and gra- 
dually pass towards the centre. When they first appear, generally the longitudinal 
but sometimes the transverse tines are most plainly marked. A few streaks are 
usually seen here and there along different portions of the fibre, and these gradually 
extend and blend together, giving to the fibre for a time an irregular streaked appear- 
ance, until at length they are seen throughout its entire substance, but for a long- 
time they remain most prominently marked towards the margins. When first formed 
they are certainly much smaller than at a subsequent period. The striae are much 
finer, and many more exist in a given space than at a more advanced period of deve- 
lopment. 
When this stage is completed, the fibre presents a very uniform appearance through- 
out its entire length, Plate XI. fig. 10. It appears as a narrow flattened band*. 
The nuclei that remain are seen at tolerably regular intervals in the substance of the 
* A form whicli in insects is often permanently maintained. — Bowman, Phil. Trans. 
It is impossible not to remark the close similarity between the appearance of striated muscular fibre at this 
period of its development and the permanent condition of the highest form of organic muscular fibre. 
