136 



REGENERA TION 



the head that regenerates from an undivided stem, yet the number of 

 tentacles on each head is more than half the average number. This 

 is connected apparently with the fact that the circumference of each 

 half is greater than half the circumference of the original stem. 

 Planarians with double tails, produced by partial splitting, have been 

 described by Duges and by Faraday, and it has also been shown 

 that by partial splitting of the anterior end of the worm two heads 

 can be produced. Van Duyne, Randolph, and Bardeen and I have 

 obtained the same result. Each half completes itself on the cut-side 

 and produces a symmetrical anterior end. If one of the heads is cut 

 off, it will be again regenerated. If the heads are united very near to 

 the trunk, as in Fig. 42, A, they may never grow to the full size of the 



B 



FIG. 42. Planaria lugubris. A. Two heads produced after operation similar to that in Fig. 24. 

 Each head about half size. B. Worm split in half through level of pharynx. New half-worms 

 larger than half of normal worm. 



original head, as I have found; but if the pieces have been split poste- 

 riorly, so that each head has a long anterior end, then each one may 

 become nearly as large as the original head (Fig. 42, B\ We see 

 in these cases the influence of the region of union on the growth of 

 the new part. If the new part is near the region of attachment, the 

 smaller size of the latter restrains the growth of the new head; but 

 if the region of union is farther distant, the head may grow more 

 nearly to its full size despite the influence of the region of union. 

 King has found in the starfish that if the arm is split lengthwise, each 

 half may complete itself laterally and a forked arm result. An addi- 

 tional entire arm may be formed by splitting the disk partially in two 

 between two arms. If the cut-edges do not reunite a new arm will 

 grow out from each cut-surface (Fig. 38, E). In this case the de- 



