The time-graded regeneration field in planarians 



sides before these have had time to start irreversibly along the head-determining 



line. 



A transverse section is cut out of a great number of Dendrocoelum; the cuts must be 

 made at the same level in the time-graded field in all specimens. The segments are 

 divided into five equal lots. In the first lot a lateral third is isolated by a longitudinal 



Figure 3. Bdellocephala 

 punctata. The time-graded 

 regeneration field; darkest 

 shading represents highest 

 regeneration rate. 



cut, as shown in Figure 4, 24 hours after the segment has been cut out of the animal. 

 The second lot is handled in the same way after 48 hours, and so on until the fifth 

 lot has had its lateral part cut away after 120 hours. 



We know that in Dendrocoelum a lateral third, when isolated, takes 7 days to regen- 

 erate a head. If an inhibitory force emanates from the median high-point, the lateral 



lateral 3 lateral 4 laterals 



7Zh O 96h A fZOh k. 



Figure 4. Transverse anterior segments of Dendrocoelum. The signs 

 are those used in the curves of Figure 5. Further explanation in text. 



piece, when isolated after a certain time, should now require more time to regenerate 

 a head. 



Figure 5 shows the results of some experiments. It is clear that an inhibition actually 

 sets in at some time between 72 and 96 hours after the segments were cut out. Lateral 

 pieces isolated after this time require more time for head regeneration than pieces 

 which were isolated during the first 72 hours. 



The distance which the inhibitory influence has to traverse is roughly about 500 

 microns; if the size of a neoblast is about 10 microns, then 50 cells transmit the 



123 



