CHAPTER XIII 

 THEORIES OF REGENERATION 



IT is significant to find that the theory of pre-formation of the 

 embryo in the egg, that was so very widely held during the seventeenth 

 and eighteenth centuries, and during the first part of the nineteenth 

 century, was at once applied to the explanation of the regeneration of 

 animals when this process became known. Bonnet in 1745 attempted 

 to explain the newly discovered facts in regard to the regeneration of 

 animals by means of the pre-formation theory. Just as the egg was 

 supposed to enclose a pre-formed germ, so he imagined there lay con- 

 cealed latent germs in the adult animal. At first Bonnet thought that 

 these germs must be whole germs, like those contained in the germ 

 cells of the reproductive organs, and that only as much of any one 

 developed as was needed to replace the missing part. Later, how- 

 ever, he admitted that the germs might be incomplete germs, which 

 are so located in each region that they represent the parts of the 

 body beyond that region. The purpose of these germs is to replace 

 any accidental injuries to the animal. He pointed out that some 

 animals are more subject to injuries than others, and these animals 

 are he thought especially well supplied with germs. Since in some 

 animals the same part may be replaced several times, Bonnet assumed 

 that on each occasion a new germ is awakened. As many sets of 

 germs are present in these animals as the number of times the animal 

 is liable to be injured in the course of its natural life. 



Bonnet found that in lumbriculus a new head and a new tail may 

 appear at almost any level, if the worm is cut in two, and, therefore, 

 he supposed, head germs and tail germs are present throughout the 

 worm. But why, if this is so, should a head germ always develop at 

 the anterior end, and a tail germ at the posterior end of a piece cut 

 from the body ? Bonnet's keen mind saw that it was necessary to make 

 a further assumption. He supposed that the fluids of the body that 

 pass forward carry nourishing substances for the head. When the 

 worm is cut in two these substances are stopped at the anterior cut- 

 surface, and there accumulating act on the latent head germ, and 

 nourishing it, cause it to develop. Correspondingly the nourishing 



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