CROSS STRIATED MUSCLE IN TISSUE CULTURES 175 



second day and do not reach their maximal gi'owth until the 

 end of the thuxl or fomlh day. The buds even at the beginning 

 of their growth appear to be less differentiated than are the fibers 

 in the explanted piece from which they grow. This is especially 

 true in the case of muscle buds from fibers where the cross 

 striations are well marked, as in the explanted pieces taken 

 from the older chicks (nine to eleven days). 



The bud first appears projecting from the edge of the ex- 

 planted piece as one or more pointed processes which adhere to 

 the cover sUp. These processes are continually changing in 

 length and size and slowly advance farther and farther out on 

 the coverslip, pulling ))ehind them, as it were, a broad thin 

 expanded mass of nmscle cytopla.sm that retains its continuity 

 with the end of one of the muscle fibers within the explanted 

 piece. As the whole mass creeps out farther, nuclei begin to 

 appear in the more proximal part of the mass (fig. 5). As the 

 large flattened protoplasmic mass creeps still farther out on the 

 cover slip, that part of the bud which connects it with the old 

 piece in many cases becomes narrower or more slender and is 

 apparently not so closely attached to the cover slip. The brush 

 like protoplasmic tips with the slender connecting fibers are well 

 shown in figure 1. The protoplasmic tips are evidently the 

 actively migratory part of the bud (figs. 1, 3, 4, 5). The proc- 

 esses are at all times more or less active. They are often long 

 and slender and usually are more numerous at the extreme end 

 of the bud than along its sides. 



As the protoplasmic end migrates farther and farther out on 

 the cover slip it apparently exerts more or less of a pull on that 

 part of the bud which connects it with the old j:)iece. It is not 

 imcommon for the resulting slender part to break in two and for 

 both ends to rapidly contract, as though the fiber had been 

 under considerable tension. The entire muscle bud may con- 

 tract back towards the explanted piece if the protoplasmic end 

 becomes loosened from the cover slip. 



There is a marked tendency for anastomoBes and fusion of 

 muscle buds either directly or by branches. The muscle buds 

 from neighboring fibers often fuse near the edge of the explant 



