188 WARREN H. LEWIS AND MARGARET R. LEWIS 



fixed specimens seem to indicate that the nuclei show all stages 

 in the process of direct division while observations on similar 

 cultures in the living fail to give evidence of a direct division. 



DISCUSSION 



The muscle buds from the explanted pieces of thie older em- 

 bryos (nine to eleven days), which arise from the cut ends of 

 the cross-striated fibers, appear to be less differentiated or more 

 embryonic in type than normal muscle fibers of the same age. 

 A process of dedifferentiation has evidentlj^ occurred in the for- 

 mation of these muscle buds from the old fibers. Is this a true 

 reversibihty or merely a breakdown with elimination or absorp- 

 tion of some of the more differentiated parts of the cytoplasm? 

 Such unstriated buds are still capable of contraction and when 

 portions of them become separated off they may undergo 

 rhythmical contractions. It is then not necessarily loss of 

 function which determined this dedifferentiation. Contractions 

 occur however rather rarely. The fibers in the old piece are 

 of course entirely severed from all nervous connections and there 

 is no indication that they contract yet they retain their cross- 

 striations. 



This process of dedifferentiation or a return to a more embryonic 

 condition probably underlies all types of regeneration. We 

 doubt if there is ever any regeneration of differentiated tissue 

 without a preliminary return of the cells involved to a more 

 embryonic condition. In regeneration this preliminary stage 

 of dedifferentiation prepares the way for growth and redifferen- 

 tiation. The dedifferentiation in regeneration does not neces- 

 sarily proceed to the extent in which the cells of the various 

 tissues return to a common embryonic type, such as Champy 

 maintains happens to practically all cells in tissue cultures. 

 As we have seen this process of dedifferentiation does not pro- 

 ceed in our cultures to such an extent as to render the muscle 

 cells indistinguishable from other types of cells. Prolonged 

 cultivation might result in a return to a still more embryonic 

 type of the outgrowing muscle tissue. 



