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examined microscopically, 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 outline is di- 

 stinct and well denned, 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 con- 

 sists in the aggregation and adhesion of the cytoblasts, and their in- 

 vestment 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 homoge- 

 neous 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 distance apart. At the same 

 time the fibre strikingly decreases in diameter ; for as the nuclei se- 

 parate, 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. 



