OF STRIATED MTJSCULAR FIBEE, 107 



The development of the muscular structure of the heart differs in some remarkable 

 particulars from those of the extremities. Three points are noticeable in the fibres of 

 the fully-formed heart : — 1, the small size of many of the fibres ; 2, the apparent 

 absence of a sarcolemma ; and 3, anastomoses between and occasional branching of the 

 fibres. 



If the heart of a Chick be examined after twenty-four hours of incubation, it will be 

 found to consist chiefly of round cells, a few fusiform having made their appearance, and 

 some stellate cells being also present. The stellate cells become more numerous after forty- 

 eight hours of incubation, and by the third day large tracts of the tissue of the heart 

 are seen to consist of these cells, which anastomose freely with one another in all direc- 

 tions*. Fig. 47 (Plate VI.) represents these cells at this period. They are most irregular 

 in size and shape, giving off" processes in all directions, which are of very variable thick- 

 ness, and which often unite with those given off' from other cells. Their contents are 

 very granular, and there are some very indistinct appearances of striation in them. 

 Their outline is very sharply defined (at least after the use of a solution of bichromate 

 of potash), and around many of them I could trace a distinct double contour. Their 

 nuclei also are very variable in size, and very irregular in position, sometimes having a 

 , situation nearly central in the cells, in other places occurring in groups upon the pro- 

 longations {h, fig. 47, Plate VI.). 



By the fourth day they form a more continuous tissue (Plate VI. fig. 48), inter- 

 lacing in all directions and giving rise to an extremely complex structure, composed of 

 Irabeculse crossing one another in all directions, in which, in parts, the origin from the 

 earlier stellate and anastomosing cells can still be seen. The nuclei are scattered very 

 irregularly over these structures. The trabeculse are all at this period very distinctly 

 striated, the striation being more marked in the longitudinal than in the transverse 

 diameters, and the tissue is extremely granular. 



The increasing complexity of structure renders it difficult to follow with any cer- 

 tainty the further development of this structure after the fourth and fifth days of incu- 

 bation ; but from the fifth to the eighth day there appear, in addition, numerous elon- 

 gated bodies, sometimes single, sometimes divided at their ends, and containing one or 

 more nuclei, dimly granular throughout, and having at these periods no traces of stria- 

 tion, and presenting in the majority of instances (except when divided at their ends) the 

 strongest possible resemblance to organic muscular fibre. They appear to be uncon- 

 nected with the interlacing network formed by the stellate cells last described, but I 

 have not been able to trace their further destination. 



Conclusions. 

 It will be seen from the foregoing description that I regard the development of mus- 



* These are too numerous, and occupy too large a relative amount of tissue, to be concerned simply in the 

 formation of nervous ganglia ; moreover tlieir subsequent development shows them to be early stages of mua- 

 cular fibre. 



