I 4 6 REGENERATION 



(iv) Can we account, in any way, for the presence of self-division 

 in certain groups, and for its absence in others ? 



(v) What relation exists between the forms that prepare for sub- 

 sequent self-division and those that do not ? 



The first question is easily answered. Regeneration is also found 

 in nearly all the other groups that do not propagate by self-division, 

 as, for instance, the mollusks, vertebrates, etc. The second half of 

 the question may also be answered. All the groups that propagate 

 by self-division have also the power of regeneration. 1 



In answer to the second question there is ample evidence showing 

 that regeneration is by no means confined to those regions of the 

 body in which the self-division occurs. 



In answer to the third question, it may be stated that although, 

 in the groups that propagate by self-division, regeneration may be 

 present in nearly all parts of the body, the same phenomenon occurs 

 in other groups that do not propagate by division. 



The fourth question offers many difficulties, and our answer will 

 depend largely upon what we mean by " accounting for" the process 

 in certain groups. If the question is interpreted to ask, Why does 

 an animal divide ? no answer can be given. If it is meant to ask, Can 

 we see how the process would be difficult, or even impossible, in cer- 

 tain groups and not in others ? then an approximate answer may be 

 given, or at least an hypothesis formed. In the first place, the power 

 of regeneration must be present in the region at which the self- 

 division takes place in order that the result may lead to the formation 

 of new individuals, or else be acquired in that region along with the 

 acquirement of the means for division. A leech is not much more 

 complicated than a marine annelid, yet it has little or no power of 

 regeneration ; hence, perhaps, propagation by division could not be 

 acquired by the leeches until they had first acquired the power to 

 regenerate. In the second place, in certain forms a separation of the 

 body into two parts would lead to the death of one or of both parts, 

 owing to the dependence of the different regions upon each other. 

 In forms like the vertebrates, insects, Crustacea, etc., we can readily 

 see why this would be the case. Hence propagation by means of 

 self-division could not be acquired, since the division itself would 

 lead to the destruction of the organism. In the third place, the 

 structure of the body may be such that the process of self-division 

 would be mechanically impossible. A hard outer coat, like that 

 of the sea-urchin, combined with a weak development of the mus- 



1 The proglottids of the cestodes seem to be an exception, hut they are little more than 

 sacs filled with embryos at the time of their separation. How far regeneration may take 

 place in the scolex, or young proglottids, is not known, but it is not improbable that some of 

 the abnormal forms that have been described may be due to regeneration. 



