292 Changes in the Mnscle Cell of Xecturus 



up into smaller spheres, which are gradually absorbed. This process 

 continues, until in the 21 mm. stage, the spheres or granules that 

 remain are very minute and often arranged in rows. If the 26 mm. 

 stage be examined after osmic acid fixation these blackened granules 

 stand out with great clearness. They are now disposed in regular linear 

 series and in correspondence with the transverse markings. These gran- 

 ules frequently lie at the surface of the myoblast and again within ; often 

 they terminate at the ends of a nucleus, appearing as if in continuity 

 with the nuclear substance. 



Similar structures have been repeatedly described in adult muscle 

 cells of various amphibia. By Kolliker, 57, they were considered as a 

 third normal element of the muscle cell. Max Schulze, 61, regarded 

 them as an undifferentiated portion of the primitive protoplasm; while 

 Weber, 74, and van Grehuchten, 89, concluded that they were patho- 

 logical structures. I have not found these structures in the adult fibre 

 of Necturus. Their structure and arrangement in the late larval stages 

 have suggested that possibly there is a close relation between them and 

 the problematic structures described by others in the adult fibres. 



The nuclei lie at the periphery of the myoblast, and for the most part 

 on the cutis side (Plate I, Fig. 12). In general form they are somewhat 

 longer than in the earlier stages, attaining an average length of about 

 47 fx and a width of 10 /a; they are also more or less flattened, as shown 

 in Plate III, Figs. 28-39, thereby bringing a much greater extent of 

 nuclear surface in contact with the cytoplasm. 



The profile view and transverse sections represented in Plate III show 

 that the nuclear membrane (n. m.) is more completely obscured by the 

 peripheral layer of karyosomes than in the earlier stages. 



The linin network {I) has undergone striking changes in character. 

 It is greatly decreased in quantity ; its threads are coarser, more regular, 

 and in general radially disposed. 



One of the most striking changes is the entire disappearance of plasmo- 

 somes which are present in both the earlier and later stages. 



The karyosomes, as shown in Plate III, Figs. 27-39, are arranged in 

 a more definite manner than in the preceding stage, being usually so 

 grouped that they form an irregular axial mass and a wide peripheral 

 band. Those in the axial mass are larger than those at the periphery 

 and are most frequently so disposed that their long axes are parallel with 

 the long axis of the nucleus. Those at the periphery are so closely ap- 

 posed that they give rise to an apparently continuous band, but this peri- 

 pheral band, instead of being of uniform width, is much thicker on the 

 side next the fibrillge. 



