1 8 2 RE GENERA TION 



planted tissues can no longer manufacture the substances necessary 

 for their specific growth, and after the materials that have been 

 brought along with them have been used up, the growth of the piece 

 is stopped and its subsequent degeneration begins. It would be 

 interesting to see if pieces transplanted to the same kind of organ 

 as that to which they belong will become permanently incorporated 

 in their new position. 



The grafting-experiments that have been described in the preced- 

 ing pages were carried out with pieces of adult organisms. Some- 

 what different conditions are present when parts of the developing 

 egg or embryo are united, inasmuch as a process has been started 

 in them that may go on independently, to a certain extent, of the union 

 of the pieces. Born has carried out a large number of experiments 

 in grafting parts of tadpoles of the same species, and also of differ- 

 ent species. The union is brought about at the time when the tad- 

 poles are about to leave the jelly membranes. The cut-surfaces are 

 brought in contact and the pieces pushed together and held in place 

 for an hour or two by means of small silver blocks or pieces of wire. 

 The pieces readily stick together, and the union is a permanent one. 

 Before describing Bern's results, it may be well to consider the power 

 of regeneration of young tadpoles. If the tail is cut off a new one is 

 regenerated by the tadpole, but all parts of the body do not have 

 this same power. Schaper found that if a part of the brain, or even 

 the entire brain, is removed, no regeneration takes place. I have 

 found that if the region where the heart is about to develop is cut 

 out from a young embryo, a new heart is not formed. 1 If a tadpole 

 is cut in two across the middle of the body, neither piece regenerates 

 the missing half. Byrnes has found, however, that if the region from 

 which the posterior limb develops is cut out a new limb regenerates. 

 In older tadpoles, Spallanzani found that if the hind limb is cut off 

 it will regenerate, and Barfurth has more recently confirmed this 

 result. The end of the tail that has been cut off from a young tad- 

 pole, before the tail has begun to differentiate, may continue alive for 

 several days. It grows larger and flatter, and the V-shaped meso- 

 blastic somites are formed. A slight regeneration even starts at its 

 anterior end, as first observed by Vulpian and later by Born. The 

 notochord and nerve-cord may send new tissue into the new part, and 

 even some of the muscle cells may extend into this part, but the piece 

 dies before regeneration goes any further. If, however, the tail is 

 grafted in a reverse direction on the body of another tadpole, the 

 regeneration may go further and produce a tail-like structure, as 

 Harrison discovered and as I have also seen. 



1 In one case I observed rhythmic pulsations in a vessel on one side of the neck, in the 

 region above the pharynx. 



