CLARKE, ON STRIPED MUSCULAR FIBRE. 229 



quence, apparently, of this arrangement, the bundles have 

 increased a little in average diameter. Some of them, how- 

 ever, are very small in this respect, and consist of not more 

 than three or four fibrillee (fig. ISb). In the majority of 

 cases the fibrils, in turn, have, either wholly or in part, become 

 resolved into a succession of granules, which, under certain 

 conditions, assume the appearance of transverse striae. Fig. 

 13 a represents one of the larger fibres, in which each of 

 these appearances is seen at different parts of its course ; and 

 b is one of smaller diameter, in which the fibrillse are only 

 here and there resolved into granules. Every fibre bears 

 on its surface a variable number of nuclei, which are 

 frequently disposed alternately and with much regularity 

 around it. From these nuclei granular processes creep along 

 the surface and begin to develop into new fibres. 



In mammalia the development of muscular fibre proceeds 

 on the same plan, and in all essential details is carried out 

 in nearly the same way as in birds. My observations were 

 made chiefly on the foetal ox and sheep, especially on the 

 latter. In the foetal sheep of half an inch in length the 

 structure of the muscular tissue has much resemblance to 

 that of the chick near the end of the fifth or at the begin- 

 ning of the sixth day of incubation. The fi'ce nuclei con- 

 tained in the blastema are rather larger, but similar in all 

 other respects. In the midst of these lie a multitude of 

 fibres, which, when undisturbed, appear to be nearly parallel, 

 and when seen under a power of about 400 diameters re- 

 semble pieces of coarse thread (see fig. 14 a). Between 

 these, and often in connection with them, finer fibres 

 are formed by a more irregular condensation of the 

 blastema, or coalescence of its granules from the sides or 

 ends of the nuclei, which therefore appear very thickly 

 clustered around them (see fig. 14 6). When the coarser 

 fibres are sufficiently isolated by dissection, their connections 

 with the nuclei, though all on the same plan, present some 

 difi"erences of appearance in different cases. At first a certain 

 amount of blastema ajJiJarently condenses all aroiind or at 

 the ends of nuclei into masses of more or less definite shape, 

 and becomes bounded, as already described, first on one side 

 and then on the other, by a distinct fibre or border. Some- 

 times the nuclei fall into a single linear series, and if they 

 happen to be pyriform, assume an imbricate arrangement, 

 so that their smaller ends or granular processes, each over- 

 laid by the next, are connected in a continuous line to form 

 a lateral band or fibre (fig. 14 c?). If the blastema condense 

 at each end of the nuclei in such a way as to constitute 



