CROSS STRIATED MUSCLE IN TISSUE CULTURES 171 



alniiulaut in quantity that they could not well be overlooked 

 if present in plasma cultures. 



THE EXPLAXTED PIECES 



The explants consist of small pieces of muscle a millimeter 

 or less in diameter taken from the muscles ot the back, wing 

 or leg of chick embryos of se^-en to eleven days incubation. 

 The muscle fibers in the explanted pieces show somewhat vary- 

 ing degrees of differentiation of the cross-striations. The gen- 

 eral character of the outgrowth, however, is much the same 

 from pieces of muscle of the above ages, although no two cultures 

 are exactly alike. 



In the seven day chick the cross striations are but slightly 

 developed in the myotomic muscles of the back and they are 

 practically not developed at all in the limb muscles. In the 

 nine day chick, however, the cross striations are very apparent 

 in the muscles of both the back and the limbs, especially the 

 muscles of the upper part of the wing and the leg. 



The explanted pieces consist for the most part of a matrix 

 of mesenchymal cells in which are embedded the young muscle 

 fibers many of which are cut across at one or both ends. Huber 

 has recently shown that in the adult rabbit muscle the fibers 

 vary greatly in length even in the same fasciculus and probably 

 the same condition holds in the young developing muscles of 

 the chick embryo. We should expect then that the fibers in 

 the piece at the time of transplantation would be of various 

 lengths. 



The variations in the size and the length of the fibers in the 

 explanted pieces would explain in part at least the great differ- 

 ence in the length and the size of the outgrowing muscles buds. 

 The medium does not, of course, afford all the necessary sub- 

 stances for growth. The muscle bud is probably derived for 

 the most part from the substance of the old muscle fiber, the 

 medium may furnish some food and the substances derived 

 from the disintegration of cells within the explanted piece may 

 also contribute. 



