The time-graded regeneration field in planarians 



In Bdellocephala no head-regeneration occurs at wound surfaces in the hindmost 

 three-fifths of the body. If a transverse segment from the forepart is bisected along the 

 median line, a posterior segment likewise, and the anterior half transplanted to a 



r\ 



S*\ 



u 



u 



V 



Figure 1 1 . Planaria lugubris. 

 After decapitation a was inter- 

 changed with b; head-regenera- 

 tion followed at the rate inherent 

 for b-level. 



Figure 12. Planaria lugubris. 



The a segment was transplanted 



to b; head-regeneration from a 



followed at the rate inherent for 



the di-level. 



posterior half, the two will build a common blastema, but only the anterior half will 

 regenerate a head, and this is so both with normal and reversed polarity (Figures 



I3-H)- 

 Isolated halves will start head-regeneration by making their 'own' half head first; 



a left half will regenerate a left eye, only later also a right eye. That is to say that the 



Figure 13. Bdellocephala. The segment a was transplanted 

 to b with the same polarity ; only a regenerates head. 



time-graded regeneration field is, in reality, two fields, a left and a right. Investiga- 

 tions into the relations of the two symmetrical fields may therefore throw some light 

 on the problem of bilaterality. 



The time-graded regeneration field in Planaria lugubris extends all over the body. 



127 



