138 GEORGE FRED SUTHERLAND. 



The similarity of the rate curves based on a counting of the 

 mitotic divisions with that based on the amount of tissue formed 

 at each period, seems to be significant. It shows that the rate of 

 tissue formation is closely correlated with the number of mitotic 

 divisions. Considering amitosis, this may be interpreted in 

 one of two ways — (i) either the number of amitotic divisions is 

 similarly correlated with the rate of growth so that the total 

 number of divisions both mitotic and amitotic, gives the same 

 form of curve as the mitotic divisions alone, or else (2) amitotic 

 divisions are not numerous enough to be significant. The former 

 explanation is improbable. The nuclear conditions producing 

 mitotic division are probably different from diose producing 

 amitotic division. Different cells in the same region may divide 

 by different methods, but it is very improbable that the conditions 

 producing one form of division would increase and decrease in 

 influence at the same rate and the same times as those producing 

 the other form. Moreover, in the present study, no examples 

 of direct division were seenexceptin the degenerating, fragmenting 

 nuclei. This similarity of the rate curve of mitotic divisions to 

 the rate curve of growth is evidence, other than the negative 

 observational evidence, supporting the view that amitotic division 

 is not important in the formation of this organ by regeneration. 



V. Summary. 



1. The regenerating spinal cord of the frog tadpole has been 

 studied histologically in order to learn the mechanism, or the 

 stages in the process, by which the new cord is formed from the 

 old. 



2. During the first day after the operation, injured nuclei in 

 the end of the spinal cord degenerate. There is first a decrease 

 in size, by contraction or loss of achromatin, and then a frag- 

 mentation of these degenerating nuclei. The fragments may be 

 carried away either by the outbreaking of a cerebrospinal fluid 

 or by leucocytes which appear at this time. These fragments 

 are parts of disintegrated spinal cord nuclei and not of leucocytes. 



3. From the second to the sixth days there is a temporary 

 partial closing of the neural tube, probably by migration of the 

 cells near the end. 



