H. V. BR0NDSTED 



We know nothing definite as yet as to the nature of the time-graded regeneration 

 field in planarians. However, a series of transplantation experiments has revealed that 

 it must have a rather firm structural base; humoral conditioning is, as far as I can 

 see, out of the question. Some of these experiments will now be mentioned. The 

 transplantation technique has already been described (Brondsted, 1939). 



U 



Figure 9. Planaria lugubris. 

 After decapitation a median part 

 is exchanged with a lateral part. 



If in Planaria lugubris the median part of the field is exchanged after decapitation 

 with a lateral part (Figure 9), these two parts will regenerate heads at the normal rate 

 characteristic of the level of the cut, irrespective of the new situation; the median 

 will not be slowed down, nor will the lateral be accelerated. An example is shown in 

 Figure 10. 



Figure 10. Planaria lugubris. The interchange 



of a and b as seen in Figure 9 does not produce 



any effect on the rate of regeneration inherent in a 



median and a lateral piece. 



If anterior transverse segments are interchanged with posterior ones in such a way 

 that the posterior pieces take up the position of the anterior ones, head-regeneration 

 from the anterior wound surface will proceed at the rate characteristic of the level at 

 which that wound was made (Figures 1 1-12). 



126 



