304 Changes in the Muscle Cell of Xecturus 



These and many other observations indicate that the position of the 

 nucleus in the muscle cell, as in other cells, is not one of an accidental 

 character, but one brought about through a precise physiological correla- 

 tion of cytoplasmic and nuclear activities. 



The changes in the structure of the nuclei may be summarized best by 

 considering separately the various nuclear elements. 



The nuclear membrane shows some interesting changes during cyto- 

 morphosis, the most notable l^eing the formation of grooves or corruga- 

 tions in the adult stages which serve to increase considerably its contact 

 surface. Chemical changes are also indicated by its decreased staining 

 properties. 



The linin network in the earlier stages (6-15 mm.) is made up of fine 

 meshes, which are more or less obscured by the numerous and widely 

 scattered karyosomes. As the muscle cell approaches the period in 

 which its cytoplasmic activity is most marked the meshes of the nucleus 

 become larger and the threads straighter and coarser, with increased 

 staining capacity. As it passes over into the adult condition the fine 

 meshwork reappears but the affinity for chromatic stains has been lost. 



The plasmosomes show interesting changes during the growth, of the 

 myoblast. In the earliest stages (6-7 mm.) but two or three are found, 

 but as the differentiation of the cytoplasm proceeds (9-10 mm.) they 

 become more numerous, four or five being usually present. With the 

 increased cytoplasmic activity of the 17-26 mm. stages these structures 

 entirely disappear, but later reappear in the old nucleus. In the early 

 stages they readily stain with any of the ordinary basic stains, but in the 

 old fibre this capacity is greatly lessened if not lost. From my observa- 

 tions I am unable to offer evidence either for or against the supposition 

 that these structures are to be regarded as by-products rather than active 

 nuclear elements. 



Concerning the arrangement of karyosomes or chromatin in the nuclei 

 of the muscle cells of ISTecturus there is a single observation to be cited. 

 Macallum, 87, -162, says : " In what might be considered as young nuclei 

 the chromatin is usually quite distinct, arranged in short, variously 

 looped pieces along the long axis of the nucleus, or in the form of 

 minute nodules (nucleoli) in different positions in the nuclear cavity.'' 



Eabl, 89, 242, describes the muscle fibres in the embryo of Pristiurus at 

 the time the first fibrillse are formed, and in speaking of their nuclei 

 says that in a very early stage they show peculiar characters; they stain 

 less intensely than the nuclei of the surrounding mesenchymal cells and 

 possess an axial rod of chromatin. 



