NUCLEAR CHANGES IN RANA CLAMITANS. I37 



his later preparations evidently show the cord again open. The 

 regenerated spinal cord at sixteen days has almost reached its 

 maximum length, but it is not yet closed. Whether or not the 

 completely regenerated spinal cord is open at the end or closed as 

 In the normal tail cannot be answered by the present study. 



4. Rate of Division. Amitosis versus Mitosis. 



Durbin (1909), in analyzing the rate of increase in length 

 throughout the regenerative process in the tail of Rana damitans, 

 distinguishes four periods. "The operation was followed by an 

 interval of low rate, succeeded by one of rapidly increasing rate, 

 then by one of rapidly decreasing rate and finally an interval in 

 which the rate gradually approaches zero. The first low period 

 is explained by a combination of two factors, (a) the shock of the 

 injury, and (b) the formation of a cap of embryonic cells which is 

 to serve as a basis for the more active regeneration. The second 

 or period of rapidly increasing growth is the one in which prac- 

 tically all the cells in the new part are undifferentiated and 

 rapidly dividing. The third and fourth periods are explained by 

 the appearance of differentiation, which lessens the number of 

 dividing cells." 



Fig. 12, based on the number of mitotic divisions in the spinal 

 cord, shows these same periods. The initial period of low rate 

 covers the first two days; that of rapidly increasing rate includes 

 the third to ninth days; the period of rapidly decreasing rate 

 extends from the tenth to sixteenth days, and the period of 

 gradually decreasing rate, though not covered in the present 

 work, would undoubtedly extend on from about sixteen days. 

 In the light of this histological study, a somewhat different inter- 

 pretation might be given to the initial period. It is during these 

 first two days that degeneration of the injured cells is taking 

 place. Though at this time a cap of undifferentiated cells is 

 being formed over the wound, the spinal cord does not participate 

 in the formation of this cap, nor is any such cap formed at the 

 end of the spinal cord. Since the spinal cord cells in this part of 

 the tail are so slightly differentiated, the new cord is formed from 

 the old without the separation of a group of special embryonic 

 cells. 



