380 



I have observed the same peculiarity. Later the Duclei loose this 

 main granule and have instead a number of smaller ones." 



J. B. Macallum states that in the human embryo of 180—170 mm 

 the muscle fibres stop increasing in number and at this thime the 

 nuclei change in character from the vesicular centrally disposed, to the 

 solid peripherally placed nuclei. Macallum suggests that there is 

 possibly a relation between the position of the nuclei and the power 

 of the cells to produce new fibres. 



While these peculiar changes in the nuclei of the muscle cell have 

 been noted by these investigators, no one, so fas as I am aware, has 

 given them further attention. 



This has been extended through the study of the same structures 

 in a number of other vertebrates which are in the Harvard Embryo- 

 logical Collection, including Petromyzon, Squalus, Amia, Batrachus, 

 Frog, Chick, Pig and Man. 



Since in most of the forms studied the phenomena are essentially 

 similar to those observed in Necturus, the description will be here 

 restricted to the changes in this form. While the changes in Necturus 

 have been followed in a series of closely related stages, it will here 

 suffice to describe three of these stages. Firstly, the structure of the 

 nucleus before fibrillation has begun. Secondly, the structure of the 



nucleus during the phases of maximal 

 cytoplasmic activity, as revealed through 

 fibrillation. Thirdly, the structure of the 

 nucleus in the adult or old animal. 



In the 6 — 7 mm stages, before fibril- 

 lation has begun, the nuclei lie scattered 

 at various levels throughout the myotome. 

 There are no indications of a regular 

 arrangement as Remak has figured and 

 described in the Frog. The nucleus of 

 one of the muscle cells rarely lies opposite 

 that of an adjoining cell. Numerous karyo- 

 kinetic figures have been observed and 

 much care has been taken to find evidence 

 of the longitudinal division of the muscle 

 cells but with negative results. Transverse 

 sections show that the nuclei at this time 

 lie in the long axes of the cells. In form 

 they vary from the oval to the more 

 common type shown in Fig. 1. 



s- n. m. 



-~p. 



k. 



Fig. 1. 



