2 REGENERA TION 



Trembley found that when a hydra is cut in two, the time required 

 for the development of the new individuals is less during warm than 

 during cold weather. He also found that if a hydra is cut into three 

 or four parts, each part produces a new individual. If these new 

 hydras are fed until they grow to full size, and are then again cut 

 into pieces, each piece will produce a new polyp. The new animals 

 were kept in some cases for two years, and behaved in all respects 

 as do ordinary polyps. 



Trembley also found that if the anterior, or head-end, with its 

 tentacles, is cut off, it also will make a new animal. If a hydra is 

 cut lengthwise into two parts, the edges roll in and meet, and in an 

 hour, or less, the characteristic form may be again assumed. New 

 arms may appear later on the new individual. If a hydra is split 

 lengthwise into four pieces, each piece will also produce a new polyp. 



If the head-end only of a hydra is split in two, each half becomes 

 a new head, and a two-headed hydra results. If each of the new 

 heads is split again, a four-headed hydra is produced ; and if each 

 of the four heads is once more split in two, an eight-headed hydra is 

 formed. A hydra of this kind, in which seven heads had been pro- 

 duced in this way, is shown in Fig. i, A. Each head behaves as a 

 separate individual, and all remain united on the same stalk. If the 

 foot-end of a hydra is split, a form with two feet is produced. 



One of the most ingenious and most famous experiments that 

 Trembley made consisted in turning a hydra inside out (Fig. i, B, I 

 and 2). The animal tends to turn itself back again, but by sticking a 

 fine bristle through the body, Trembley thought that the turning back 

 could be prevented, and that the inner surface of the hollow body 

 remained on the outside, and the outer surface of the body came to 

 line the new central cavity. Each layer then changed, he thought, 

 its original characteristics, and became like that of the other layer. 

 The details of these experiments will be described in a future chapter, 

 as well as more recent experiments that have put the results in quite 

 a different light. 



Reaumur repeated Trembley's experiment of cutting a hydra into 

 pieces, and obtained the same results. He found also that certain 

 fresh-water worms, as well as the terrestrial earthworm, regenerated 

 when cut into pieces. At his instigation two other naturalists 1 

 examined the starfish and some marine polyps, and they concluded 

 that it was highly probable that these forms also could regenerate. 

 Reaumur pointed out that regeneration is more likely to occur in 

 fragile forms which are more exposed to injury. 



Bonnet's experiments were made on several kinds of fresh-water 



1 Guettard and Gerard de Villars. Bernard de Jussieu also, who demonstrated that star- 

 fish can regenerate. 



