REGENERATION 161 



divided into several parts by a series of transverse cuts. The 

 original polarity is retained by each of these parts, and each 

 of them will regenerate a head at its front edge, and a tail at 

 its hind edge. Only in exceptional cases can this inherent polarity 

 of a fragment be suppressed or reversed by certain treatments. 



In some cases, however, the situation is more complicated. 

 In earthworms, such as Eisenia foetida, a number of regions 

 with different powers of regeneration can be distinguished 

 (Gates, 1950). In the foremost six segments of the body, a 

 head is regenerated at a front edge, but no regeneration takes 

 place at a hind edge. In the succeeding zone of about 11 

 segments, heads are formed by both front and hind edges. 

 From segment 21 to segment 34, a head or a tail may be formed 

 at either edge. Next comes a region, extending to segment 54, 

 in which only tails are regenerated at both edges. Finally, m 

 the zone behind segment 54, a tail is formed by a hind edge, 

 but no regeneration occurs at a front edge. 



It is important to study the nature of the regeneration 

 process somewhat more closely. We have already mentioned 

 that the formation of a regeneration-bud is the first visible 

 consequence of the transection. This consists of cells of a very 

 indifferent character. The origin of this material is not in all 

 cases sufficiently well known. In some lower organisms, un- 

 differentiated cells are present at various places in the body. 

 After transection, these migrate into the wound area, and 

 accumulate there. Miss Dubois (1949), for instance, has demon- 

 strated this in the case of Planaria. Regeneration can be sup- 

 pressed here by irradiation with X-rays; the irradiated animals 

 succumb after a few weeks. But if only the foremost two fifths 

 part of the body is irradiated, and the head is then amputated, 

 regeneration does occur, though it begins a month later than 

 in non-irradiated animals. Miss Dubois has been able to prove 

 that this is due to the fact that the regeneration cells, or 

 neoblasts, migrate from the posterior, non-irradiated part of 

 the body into the wound area. In the course of this migration, 

 they must traverse the irradiated zone. The wider this zone is, 

 the more regeneration is retarded. This migration is not 

 spontaneous ; it takes place only after a wound has been made, 



Raven - Outline Physiology 11 



