300 Royal Society. 



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. 

 They increase in size ; their outline becomes indistinct ; a bright 

 border appears immediately within their margin ; their contents be- 

 come decidedly granular ; their outline is broken and interrupted ; 

 and presently an irregular cluster of granules is all that remains, and 

 these soon disappear. 



It sometimes happens that the nuclei perish while in contact, be- 

 fore the fibre elongates ; but the subsequent changes are the same. 



The strise generally first become visible at this period, imme- 

 diately within the margin of the fibre. 



The fibre is subsequently increased in size, and its development is 

 continued by means of the surrounding cytoblasts. These attach 

 themselves to its exterior, and then become invested by a layer of the 

 surrounding blastema. Thus, as it were, nodes are formed at inter- 

 vals on the surface of the fibre. These invested nuclei are at first 

 readily detached, but they soon become intimately connected and in- 

 definitely blended with the exterior of the fibre. All its characters 

 are soon acquired, the nuclei at the same time gradually sink into 

 its substance, and an ill-defined elevation, which soon disappears, is all 

 that remains. 



Lastly, the substance of the fibre becomes contracted and con- 

 densed. The diameter of a fibre towards, or at the close of intra- 

 uterine life, is considerably less than at a much earlier period. 



