4 RE GENERA TION 



tail continues to grow longer, new segments being intercalated just 

 in front of the end-piece that contains the anal opening. In summer 

 the regeneration of a new part takes place in two to three days ; in 

 winter in ten to twelve days, this difference not being due to the time 

 of year, but to the temperature. Bonnet found that if a newly 

 regenerated head is cut off, a new one regenerates, and if the second 

 one is removed, a third, new one develops, and in one case this oc- 

 curred eight times : the ninth time only a bud-like outgrowth was 

 formed. In other cases a new head was produced a few more times, 

 but never more than twelve. He thought that the capacity of a part 

 to regenerate is in proportion to the number of times that the animal 

 is liable to be injured under natural conditions. 



Bonnet found that short pieces from the anterior or posterior end 

 of the body failed to regenerate, and usually died in a few days. 

 Occasionally two new heads appeared at the anterior end of a piece 

 (Fig. i, E\ and sometimes two tails at the posterior end. 



Another kind of fresh-water worm 1 was found that gave a very 

 remarkable result. If it was cut -in two pieces, the posterior piece 

 produced at its anterior end, not a new head, but a new tail. Thus 

 there is formed a worm with two tails turned in opposite directions, 

 as shown in Fig. i, F, F' . 



Spallanzani made many experiments on a number of different 

 animals, but unfortunately the complete account of his work was 

 never published, and we have only the abstract given in his Prodromo 

 (1768). He made a large number of experiments with earthworms 

 of several kinds, and found that a worm cut in two pieces may pro- 

 duce two new worms ; or, at least, that the anterior piece produces a 

 new tail, which increases in length and may ultimately represent the 

 posterior part of the body ; the posterior piece, however, produces 

 only a short head at its anterior end, but never makes good the rest 

 of the part that was lost. A short piece of the anterior end fails to 

 regenerate ; but in one species of earthworm, that differs from all 

 the others in this respect, a short anterior piece or head can make a 

 new tail at its posterior end. 2 Spallanzani also found that if much of 

 the anterior end is cut off, the development of a new head by the 

 posterior piece is delayed, and, in some species, does not take place 

 at all. 



If a new head is cut off, another is regenerated, and this occurred, 

 in one case, five times. If, after a new head has developed, a por- 

 tion only is cut off, the part removed is replaced, and if a portion of 

 this new part is cut off it is also regenerated. If a worm is split 



1 An annelid of unknown species. 



2 This statement of Spallanzani's I interpreted incorrectly ('99), thinking that he obtained 

 a two-tailed form as had Bonnet. 



