DEVELOPMENT OF MUSCULAR FIBRE IN MAMMALIA. 657 
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 outline is dis- 
tinct 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. 
The first stage in the development of striated muscular 
fibre consists in the aggregation and adhesion of the cyto- 
blasts, and their investment by blastema so as to form 
elongated masses. In these clusters the nuclei have, at first, 
no regular arrangement. Almost, if not quite, as soon as the 
cytoblasts are thus aggregated, they become invested by the 
blastema, and this substance at the same time appears 
to be much condensed, so that many of the nuclei become 
obscured. 
These nuclei, thus aggregated and invested, next assume 
a much more regular position. They fall into a single row 
with remarkable uniformity, and the surrounding substance 
at the same time grows clear and more transparent, and is 
arranged in the form of two bands bordering the fibre and 
bounding the extremities of the nuclei, so that now they 
become distinctly visible. They are oval, and form a single 
row in the centre of the fibre, closely packed together side by 
side, their long axes lying transversely, and their extremities 
bounded on either side by a thin, clear, pellucid border of 
apparently homogeneous substance. 
It is to be observed how closely the muscular fibres of 
mammalia at this period of their development resemble their 
permanent form in many insects. 
The fibres next increase in length and the nuclei separate. 
Small intervals appear between them. The spaces rapidly 
widen, until at last the nuclei lie at a very considerable dis- 
tance apart. At the same time the fibre strikingly decreases 
in diameter; for as the nuclei separate, the lateral bands fall 
in and ultimately coalesce. 
This lengthening of the fibre and consequent separation of 
the nuclei is due to an increase of material, and not to a 
stretching of the fibre. 
Soon after the nuclei have separated some of them begin 
to decay. They increase in size ; their outline becomes 
indistinct ; a bright border appears immediately within their 
margin ; their contents become decidedly granular ; their 
