STRIATED MUSCULAR FIBRE IN MAMMALIA. 
253 
contact, as if the same number of nuclei which would have attaehed themselves to 
the fibre had it elong-ated. are now crowded together into the smaller spaee*. 
From the period of its first formation, the substance of the fibre gradually increases 
in strength and firmness as development advanees. This fact is well illustrated in 
fibres which have been preserved some time in different fluids. The more advanced 
the development of the fibre, the less readily is its texture destroyed ; therefore it is 
much more difficult to preserve the fibres in their earlier period than in the later 
stages of development. If fibres which are elongating, and in which the nuclei are 
separating, are kept for some time in a preservative liquid, and by-and-by examined, 
it will be noticed, that while the structure of those fibres in whieh the nuclei have 
separated to a considerable extent is still perfect and distinct, the substance of those 
in which the nuclei are still in contaet is broken up and confused ; and it will be 
observed that there is a close correspondence between the extent to which the nuclei 
are separated and the integrity of the fibre. 
The rate of development of different fibres in the same muscle is by no means 
uniform, and some fibres are doubtless formed at a later period than others. If, for 
instance, the muscular tissue of a pig, between 2 and 3 inches long, be examined, 
many fibres will be found in very different degrees of development. The majority 
are in the second, or passing from the second into the third stage, presenting the 
single row of closely arranged nuclei bordered by lateral bands, while others will be 
seen in the first stage of formation as elongated groups of nuelei, partially invested 
by blastema. In any specimen numerous fibres will generally be seen in various 
degrees of development within certain limits. Even in the same fibre in different 
portions of its length, this variation may often be detected (see Plates X. and XI. 
figs. 14, 8 and 9g^and i), and in fibres at a more advaneed period of development, and 
even for some time subsequently to birth, a considerable difference in size may be 
often observed. Generally speaking, there appears to be more uniformity in the 
rate of development of different muscles than of different fibres of the same muscle. 
Of the development of the sarcolemma I have little or nothing to say, except that 
according to my observation it is not formed in the manner originally described by 
Schwann and more recently by Kolliker. 
I have never been able to determine satisfactorily the exact period of its first 
appearance, nor do I imagine that it is possible to do so. It is probably formed 
gradually, and only attains its more perfect characters at an advanced period of 
development. 
To obtain a clear idea of the structure of foetal fibres, and of the changes that 
occur at different periods of their development, specimens should be examined both 
* Perhaps this was the case in the instance which Mr. Bowman has alluded to and represented. It was a 
specimen taken from the chrysalis of a tiger-moth. — Phil. Trans. 1840. 
2 M 2 
