290 Changes in the ]\Iuscle Cell of tectums 



formation of the septa and endomysiuni, Ijut the l)eginnings are now- 

 indicated by the migration of a large nnmber of mesenchymal cells into 

 the spaces between the ends of the adjoining myotomes, as shown in 

 Fig. 3 (mes.). Tliese cells emanate from "two sources; the greater num- 

 ber are derived from the peripheral mesenchyme; in addition to these a 

 considerable nimil^er come from the axial mesenchyme, as described by 

 Maurer, 92, in Siredon. These cells not only give rise to the septa, but 

 also wander in among the myoblasts and eventually invest each myoblast 

 with a more or less complete sheath of connective tissue which forms a 

 part of the sarcolemma or becomes intimately associated with it. 



Many karyokinetic figures are now observed in and among the muscle 

 fibres; these are in part those of the mesenchymal cells, and may have 

 their spindles in almost any plane. It is therefore necessary to use con- 

 siderable care in the interpretation of the various figures lest they be con- 

 fused with the nuclei of the myoblasts, in which the planes of division 

 are always parallel with the long axis of the cell. 



Through the absorption of the yolk, the character of the cyto-retic- 

 ulum is now clearly revealed, so that its relation to the fibrillse is more 

 readily determined. Various methods have been employed to bring this 

 reticulum into prominence, especially Kolossow's osmic acid method. 

 The meshes of the reticulum, as shown in Fig. 9, are exceedingly variable, 

 and bear no fixed relation either in form or size to the fibrillse. 



The structure of the nuclei at this time is not widely different from 

 that described and figured in the 6-7 mm. embryo. Since the changes 

 taking place are more pronounced in the 15-17 mm. larvae, their further 

 description is here unnecessary. 



Necturns 15-17 mm. — The myotome, which has now (Fig. 6) attained 

 a length of about 0.5 mm., appears quite unlike that of the earlier stages. 

 This change has been brought about hy the continued invasion of mesen- 

 chymal cells. These cells have sent out long cytoplasmic processes which 

 have so intertwined that they form a distinct connective-tissue sep- 

 tum (s). In the earlier stages these mesenchymal cells contained large 

 nuclei with a small quantity of cytoplasm and were actively migratory; 

 they now contain small nuclei with greatly elongated cytoplasmic pro- 

 cesses and are no longer migratory. With little difficulty one can follow 

 all the intermediate stages and see that the connective-tissue fibrils are 

 the elongated cytoplasmic processes of the previously wandering cells. 

 The yolk granules have been rapidly absorbed, until but few are present 

 in the c}^oplasm. Pari passu with the absorption of the yolk the 

 fibrillse have greatly increased in number, as shown in Fig. 11. so that 

 one-half or more of the cell is fibrillated. 



