PROCESS OF REGENERATION 



153 



experimental purposes. The cause of the decrease in regenerative 

 power appears to be that the cells at the cut surface fail to grow 

 out with their usual vigor. The rate of x is thus lowered, as 

 required in the experiment, and the expected decrease in the 

 percentage of normal heads occurs. In table 8, a comparison is 

 made between the regenerative capacity of ab pieces cut on April 

 24, and two lots cut on May 30, from the same stock. Fifty 

 pieces were taken in each case, and the results are expressed 

 in percentages. 



TABLE 8 



The decrease in normal heads is very characteristic, also the 

 appearance of biaxial tails. The number of normal heads may 

 be considerably increased in these pieces by putting them in 

 dilute cyanide. The mortality in these pieces cut late in the 

 season is unfortunately always high, but as the decrease in normal 

 heads has been noticed in every experiment performed with worms 

 kept some time in the laboratory, it cannot be accounted for on 

 the basis of the increased mortality. 



The role of the axial gradient in morphogenesis has not perhaps 

 been sufficiently emphasized. In the case of short pieces, the 

 dynamic conditions developed after section are the principal 

 factors, yet it should be obvious from what has been said that 

 these conditions are dependent on the previous position occupied 

 by the pieces in the axial gradient of the intact animal. In long 

 pieces, the axial gradient is more directly concerned; these pieces 

 are only slightly stimulated by section, and the axial gradient is 

 preserved in them in practically its original state. Therefore 

 the anterior end of long pieces always has the highest rate in 

 the piece, and the region x is never threatened with inhibition 

 from the parts behind it. For this reason, long pieces of Planaria 

 and Lumbriculus always give rise to normal heads. It is the 



