CROSS STRIATED MUSCLE IN TISSUE CULTURES 173 



In many of the cultures, owing to extensive migration, from 

 the explanted piece, of the mesenchymal cells during the first 

 two days the explanted piece often becomes thinned out so 

 that one can observe the muscle fibers within it more clearly 

 than at the time of the fii'st appearance of the muscle buds. In 

 such cases the direct continuity of the muscle buds with the old 

 fibers is definitely demonstrated m the living cultures and this 

 continuity can also be observed after the culture is fixed and 

 stained. Sections through the explanted piece and the culture 

 in a plane parallel to the cover slip likewise show this continuity 

 of the old muscle fibers and the new muscle buds. 



THE GEXERAL CHARACTER OF THE .MUSCLE OUTGROWTHS 



The muscle outgrowths, though somewhat varied in details, 

 have on the whole certain general characteristics that enable 

 one to readily distinguish them from other tissues which grow 

 out from the explanted piece (figs. 1, 2, 3, 4, and 5). 



The muscle outgrowths occur either in the form of muscle 

 buds that are continuous with the cut ends of the muscle fibers 

 or as free fibers which wander out into the medium among the 

 mesenchyme cells on the under surface of the cover slip. In 

 most cultures both the attached buds and the free wandering 

 fibers are found in abundance. The muscle buds vary in size 

 from short, slender, pointed processes to large flat masses with 

 many processes at the peripheral end and many nuclei. In 

 practically all of the cultures the outgrowth of the mesenchymal 

 cells begins earlier than that of the muscle fibers and forms a 

 considerable zone of cells about the explanted piece before the 

 muscle buds appear. 



The muscle buds usually begin to appear around the edge of 

 the explanted piece at the end of the first day or during the 



Fig. 1 Muscle from the leg of a seven day chick embryo cultivated in ? 

 Locke's solution plus i bouillon plus 0.5 per cent dextrose for forty-eight hours. 

 Osmic acid vapor fixation, Benda stain. The long muscle buds radiate out 

 from the explanted piece and are easily distinguishable from the mesenchyme 

 cells. The explanted protoplasmic end of the muscle buds contain many nuclei. 

 The muscle buds show branches and anastqmoses. X 100. 



