HISTOGENESIS OF STRIATED MUSCLE 



8 9 



outer or myogenetic plate of the myomeres, each of which consists of a 

 solid mass of embryonic cells with polygonal sides, fitting closely to one 

 another and not differing greatly in appearance from the generality of 

 other embryonic cells around them. They are rapidly multiplying at this 

 tune, as is demonstrated by the numerous mitotic figures that are to be 

 be seen (Fig. 86). 



The first evidences of muscular differentiation are most marked. 

 The cells on the outer edge of the myotome begin to lengthen, pushing 



FIG. 86. A very young myotome of an embryo of the sucker, Catostomus. A, region of 

 mitotic multiplication of sarcoblasts; B, region of amitotic multiplication of nuclei in 

 young muscle cells. 



out their cell bodies in a line parallel with the main axis of the fish's 

 body. They continue this lengthening until they reach from one end of 

 the myotome well into the mass of cells toward the other end. Two 

 other features accompany this lengthening process; the cell becomes 

 much larger in bulk and its nucleus changes much in character, becom- 

 ing larger and oval, while its nucleoli enlarge. The staining reaction of 

 the nucleus is very different at this time : the chromatin is spread out in 

 more definite masses and the nucleolus loses its affinity for iron haema- 

 toxylin. It will still stain with it, but in the decolorizer it loses the 

 intense black color long before the nucleoli of the undifferentiated cells 

 do (see Fig. 86). It should be noticed here that the tissue represented 

 in Figure 87 is more deeply stained (i.e. less decolorized) than that 

 shown in Figure 86. 



