STRIATED MUSCULAR FIBRE IN MAMMALIA. 
245 
of the tissue immediately beneath the surface by the side of the vertebral column of a 
foetal pig, from 1 to 2 inches in length, be examined, there will be seen, besides blood- 
corpuscles in various stages of development, nucleated cells and free nuclei, or 
cytoblasts scattered through a clear and structureless blastema in great abundance. 
These cytoblasts vary in shape and size ; the smaller ones, which are by far the most 
numerous, being generally round, and the larger ones more or less oval. Their out- 
line is distinct and well-defined, and one or two nucleoli may be seen in their interior 
as small, bright, highly-refracting spots ; the rest of their substance is either uniformly 
nebulous or faintly granular, Plate X. fig. I. 
The first stage in the development of striated muscular fibre consists in the aggre- 
gation and adhesion of these cytoblasts, and their investment by blastema so as to 
form elongated masses. In these clusters the nuclei are not at first generally arranged 
in a single series; but two, three, or even more occasionally lie side by side in appa- 
rent disorder. Almost, if not quite as soon as these cytoblasts are thus aggregated 
into these long masses, they become invested by tbe blastema, and this substance at 
the same time appears to be considerably condensed, so that the outlines of the 
nuclei become almost or completely obscured. The fibre thus appears to be irregu- 
larly cylindrical or somewhat flattened. It is so opake that its interior is no longer 
to be plainly discerned, and its surface is rough and uneven. The appearance of the 
muscular fibres at this early period of their development is represented in Plate X. 
fig. 2. It often happens, that here and there, where a nucleus has not become com- 
pletely invested by condensed blastema, a portion of its well-defined dark margin 
may be observed standing out in the circumference of the mass. These early fibres 
measure about so^oo th of an inch in diameter. It is almost impossible to estimate 
their length, as they are so readily broken in being prepared for the microscope ; hence 
the lengths of the masses presented to view vary exceedingly. It has been said that 
the cytoblasts become invested by a layer of apparently condensed blastema, imme- 
diately after or at the same time that they aggregate and adhere together. In some 
cases, before the nuclei come into contact, this external investment may already be 
discerned forming around them, giving to them occasionally the appearance, if not 
very carefully examined, of nucleated cells, and this perhaps may have led to the 
original description of Valentin. But this investment of the nuclei previously to 
their aggregation is not common : as a very general rule, the nuclei meet each other 
free as cytoblasts. 
In order to obtain a fair view of these early fibres, a portion of tissue should be 
selected from a perfectly fresh specimen, for if the embryo be kept longer than a few 
hours, the masses break up and disappear ; and this disintegration is much accele- 
rated by placing them in water, for the investing blastema appears to be readily 
dissolved and the nuclei separate. Indeed the tissue is so moist at this period, that, 
when prepared for the microscope, the addition of water is scarcely necessary. 
The action of the weaker acids also rapidly dissolves the investing blastema and sets 
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