HISTOLOGY 



Immediately that this change has set in, it will be seen that the 

 nuclei begin to rapidly divide. But not mitotically, as before. They 

 perform a perfect amitotic division that greatly increases the number 

 of nuclei without dividing the cell body. This latter grows longer until 

 it reaches from the anterior to the posterior boundary of the myotome, 

 while its numerous nuclei are stretched in a single row from one end to 



the other (Fig. 87). It is 

 worth noticing that the 

 oldest cells are always on 

 the outer edge or layer of 

 the myotome and hold that 

 position through life. We 

 find the outer muscle cells 

 or sarcoblasts beginning to 

 perform the next step in 

 development before the in- 

 nermost ones have changed 

 from mononucleated, em- 

 bryonic cells into the 

 elongating and multinu- 

 cleated sarcoblasts. 



This next step consists 

 of the formation of the 

 striated muscle fibrils in the 

 cytoplasm of the sarcoblast 

 (Fig. 87). The fibrils ap- 

 pear gradually and are faint 

 at first, slowly growing 



FIG. 87. A later stage than Fig. 86 in the development 

 of muscle tissue in the sucker, Catostomus. Fibrils be- 



ginning to form in the outer cells. No connective tissue more distinct as they de- 

 has, as yet, moved in between the muscle cells. Abun- j TU~ J-l : : 



dant amitotic division of the nuclei. Vel P' The dark aniSOtroplC 



portions are to be seen first, 



and, from the length and arrangement of these segments, as compared 

 with those of fully developed muscle, it seems that they are laid down 

 and appear as fully extended fibrils from the very first. They must, of 

 course, contract with the rest of the muscle, although, being nearest 

 the center of the body in their weakest stages, they probably do not 

 have to exert the contracting force to the degree that the fibrils out- 

 side of them do. It should be noticed here that the sarcoblast can 

 contract before the formation of fibrils and that they are earliest 

 mature, and first provided with fibrils that enable them to contract 

 more strongly, on the outer edge of the body, where the strength can 

 be used to greatest advantage (Figs. 86 and 87). The fibrils appear 

 always in one side, the inner side of the cell. The nuclei are, in 



