PROCESS OF REGENERATION 127 



Within a day or so, new tissue begins to grow out from both cut 

 surfaces of the jiiece. The anterior outgrowth develops faster 

 than the posterior one, but when it has reached a certain length, 

 its growth ceases. Its tip forms a prostomium; mouth, brain 

 and pharynx differentiate within it, and lastly the bundles of 

 setae appear in order, according to the usual law of antero- 

 posterior development. Meantime, the posterior outgrowth 

 has continued its development; within a few days an anus is 

 established, and in front of this anal segment new segments con- 

 tinue to be formed indefinitely but with gradually decreasing 

 rate if no food is given. 



Anterior regeneration occurs in a very definite manner in 

 oligochaetes. If the number of segments removed is less than 

 the number of head segments, then only those removed are re- 

 placed. If more than the head segments are remo\'ed, then the 

 head segments only are regenerated. This fact is not at all 

 remarkable, although it seems to have been so regarded, since 

 it is true of all forms in which regeneration of the anterior end 

 occurs. If pieces are taken from the body of a flatworm, or the 

 stem of a hydroid, the head regenerates directly at the cut ante- 

 rior surface, and the intermediate parts are not replaced. What 

 really happens is the reorganization of the rest of the body to fit 

 the new head. This reorganization is more extensive in the 

 oligochaetes than in the lower forms, owing to their higher degree 

 of morphological differentiation. The most noticeable changes 

 in these animals are the transformation of the old intestine, 

 which adjoins the regenerated pharynx, into an oesophagus, 

 and the development of the 'hearts' characteristic of the 

 oesophageal region. Histological examination would doubtless 

 reveal many other, changes, but I have not attempted to follow 

 the regenerative processes histologically. The normal morpho- 

 logical features of the species are thus restored, even to the for- 

 mation of the sex organs in parts of the body where they would 

 never have appeared ordinarily (Janda, '12, on Criodrilus, and 

 Mrazek, '06, on Lumbriculus). Parts anterior to the level of 

 section are never restored unless a head, or an approach to a head 

 is regenerated at the cut surface. 



