DOUBLE STRUCTURES 137 



velopment of the new arm cannot be accounted for on the assump- 

 tion that the typical form completes itself, since a sixth arm cannot 

 be supposed to be a typical structure in the starfish. The result must 

 depend on other factors, such as the presence of an open surface in a 

 region where the cells have the power of making new arms. 



Barfurth has been able to produce a double tail in the tadpole by 

 the following method : A hot needle is thrust into one side of the 

 tail, so that the notochord and the nervous system are injured. The 

 tail is then cut off just posterior to the region injured by the needle. 

 A new tail grows out from the cut-end, and also in some cases an- 

 other tail grows out at the side where the notochord was injured by 

 the needle. The injury to the notochord and the removal of tissue 

 immediately about it leads to a proliferation of cells, around which 

 other tissues are added and the new tail produced. 



Lizards with double tails have often been described, 1 and it now 

 appears that all these cases are due to injuries to the normal tail. 

 Tornier has succeeded, experimentally, in producing double and 

 even triple tails. If the end of the tail is broken off, and the tail 

 is then injured near the end, two tails may regenerate, one from 

 the broken end and one from the region of injury (Fig. 43). Under 

 natural conditions this might occur if the tail were partially bitten off 

 and the end of the tail lost at the same time. A regenerated tail may 

 produce another tail if it is wounded. A three-tailed lizard may be 

 made by cutting off the tail and then making two injuries proximal 

 to the broken end. Two of the new tails may be included in the 

 same outer covering if they arise near together, as shown in Fig. 

 43, B. Lizards with two or three tails may be produced in another 

 way. If the tail is cut off very obliquely, so that two or three verte- 

 brae are injured, there arises from each wounded vertebra a cartilagi- 

 nous tube that forms the axis of a new tail. Tornier thinks that 

 the regeneration is the result of overnourishment of the region where 

 the injury has been made, but this does not seem in itself a sufficient 

 explanation. Tornier has also been able to produce, experimentally, 

 double limbs in Triton cristatus in the following way : The limb is 

 cut off near the body, and, after the cut-end has formed new tissue, 

 a thread is tied over the end in such a way that it is divided into two 

 parts. As the new material begins to bulge outward it is separated into 

 halves by the constricting thread, and each part produces a separate 

 leg (Fig. 43, D\ The soles of the two feet in the individual repre- 

 sented in Fig. 43, D, are turned toward each other. The femur is 

 bifid at its outer end, and to each end the lower part of one leg is 

 attached. The bones in this part are fused together at the knee, so 

 that only the foot portions can be separately moved. 



1 See Fraisse for literature. 



