THEORIES OF REGENERATION 26 1 



substances for the tail flow backward, and accumulating at the pos- 

 terior cut-surface awaken a tail germ to activity. 



The part of the body in which these nourishing substances are 

 supposed to be produced is not specifically stated, but in one passage 

 Bonnet says that the fluids that flow toward the head are there 

 used up in that organ, and we may infer that he held a similar view 

 for the posterior region. He offers no explanation of the cause of 

 the flow of these substances in a given direction, and in this respect 

 his hypothesis lacks support where it is most needed. In fact, it is 

 no more improbable that a head germ should always develop at the 

 anterior end and a tail germ at the posterior end, than that head- 

 forming substances should flow in one direction and tail-forming in 

 another. It is not that it is worth while to object to Bonnet's hypothe- 

 sis on the ground that it does not explain everything, but it is worth 

 while to point out that it gives only the appearance of an explanation, 

 and that it begs the whole question by the assumption of particular 

 nourishing fluids flowing in definite directions. So far as the blood 

 is concerned, we know that the different parts of the body take from 

 it those substances or fluids that they make use of, not that special 

 fluids flow to particular regions. It is probable that Bonnet thought 

 of the blood rather than of any other subtler fluids passing through 

 the tissues; and, if so, there is nothing that we know in regard to the 

 behavior of the blood that lends support to Bonnet's idea. 



Bonnet takes care to state that the pre-formed germs may not 

 appear to us like miniature copies of the part into which they develop, 

 but they are so constructed that, as they absorb nourishment and 

 become larger, they assume a characteristic form. 



Weismann, who has also accepted the pre-formation hypothesis to 

 account for the development of the egg, has applied the same concep- 

 tion of pre-formation to the process of regeneration. He believes that 

 partial, latent germs are present in different parts of the body, and 

 that these germs are present especially in animals that are liable to 

 injury and in those parts of the body that are likely to meet with 

 accidents. In these essential respects, Weismann's idea is the same 

 as Bonnet's; but in regard to the location of the germs, and their man- 

 ner of awakening, and as to how the forms, liable to injury, have ac- 

 quired their power to regenerate, Weismann adopts more modern 

 standards. He believes that the germs are located in the nucleus. 

 Those that bring about the development of the egg are supposed to 

 be different from those that bring about regeneration, because the 

 method of regeneration is generally different from the method of 

 development of the egg. 



Regeneration, on Weismann's view, is brought about by latent 

 cells containing pre-formed germs in the chromosomes of the nucleus. 



