REGENERATION IN ANNELIDS
183
stantiate the fact that a small fraction of the nerve chain in the region
of the wound is enough to stimulate the regenerative process, but that
it is necessary for the nerve chain to be uninjured throughout its entire
length for regeneration to be normal. Finally, this quantitative function
could also explain the observations made by Abeloos and Thouveny
(1960) in the polychaete Magalia perarmata where there is a cephalic
regeneration gradient which is directly related to the decreasing volume
of the nervous system enclosed in the epidermis.
To conclude, it is admissible that in annelids, the nervous system is
indispensable to cephalic or caudal regeneration; it does not, however,
necessarily have to be represented in the region of the scar by the
severed end of the nerve cord ; the presence of regenerating nerves will
suffice. The number of nerve fibres growing into the blastema will
affect its development and consequently the final form of the regenerated parts.
Interaction of the nerve fibres and the scar tissue has recently been
analysed carefully by Avel and Rozier (unpublished: see Avel, 1961)
and by Kawakami (1961). In these experiments the nervous system is
left in place in the initial stages of cephalic regeneration and removed
later on.
Avel and Rozier worked on Eisenia foetida unicolor. As we have
already mentioned, in this variety of Eisenia, regeneration, as judged by
external appearance, is completely inhibited in the absence of the nerve
cord. Avel and Rozier cut off eight anterior segments, then, after an
interval varying from 2 to 10 days, they removed the nerve chain from
the next two to four segments. If the regenerating part had been
innervated for only 2 days, the ectomesodermal blastema stopped
developing: only the oesophagus was regenerated, making its way
backwards (Fig. 21, 1). After innervation for 3 days there was a very
limited amount of cephalic regeneration in some exceptional cases.
After innervation for 4 days, a slightly protruding regeneration was
observed which contained all the internal organs ; it should be noted that
the nervous system and the pharynx were normally though slowly reconstructed. If innervation lasted for 5 days, a typical head formed, but
with fewer segments than in normal regeneration. In these cases
orientation of the developing organs becomes difficult, as is illustrated
in the case of the pharynx in the regeneration zone shown in Fig. 21,2.
Finally, if the nerve chain was not removed until after 10 days (at
which time the regenerated part contained internal organs in full
differentiation), regeneration proceeded normally. From their experiments, Avel and Rozier concluded that the nerve chain is essential for
the formation of a regenerative blastema during the first 3 days following transection. From the 4th day the blastema essential for the con-
183
stantiate the fact that a small fraction of the nerve chain in the region
of the wound is enough to stimulate the regenerative process, but that
it is necessary for the nerve chain to be uninjured throughout its entire
length for regeneration to be normal. Finally, this quantitative function
could also explain the observations made by Abeloos and Thouveny
(1960) in the polychaete Magalia perarmata where there is a cephalic
regeneration gradient which is directly related to the decreasing volume
of the nervous system enclosed in the epidermis.
To conclude, it is admissible that in annelids, the nervous system is
indispensable to cephalic or caudal regeneration; it does not, however,
necessarily have to be represented in the region of the scar by the
severed end of the nerve cord ; the presence of regenerating nerves will
suffice. The number of nerve fibres growing into the blastema will
affect its development and consequently the final form of the regenerated parts.
Interaction of the nerve fibres and the scar tissue has recently been
analysed carefully by Avel and Rozier (unpublished: see Avel, 1961)
and by Kawakami (1961). In these experiments the nervous system is
left in place in the initial stages of cephalic regeneration and removed
later on.
Avel and Rozier worked on Eisenia foetida unicolor. As we have
already mentioned, in this variety of Eisenia, regeneration, as judged by
external appearance, is completely inhibited in the absence of the nerve
cord. Avel and Rozier cut off eight anterior segments, then, after an
interval varying from 2 to 10 days, they removed the nerve chain from
the next two to four segments. If the regenerating part had been
innervated for only 2 days, the ectomesodermal blastema stopped
developing: only the oesophagus was regenerated, making its way
backwards (Fig. 21, 1). After innervation for 3 days there was a very
limited amount of cephalic regeneration in some exceptional cases.
After innervation for 4 days, a slightly protruding regeneration was
observed which contained all the internal organs ; it should be noted that
the nervous system and the pharynx were normally though slowly reconstructed. If innervation lasted for 5 days, a typical head formed, but
with fewer segments than in normal regeneration. In these cases
orientation of the developing organs becomes difficult, as is illustrated
in the case of the pharynx in the regeneration zone shown in Fig. 21,2.
Finally, if the nerve chain was not removed until after 10 days (at
which time the regenerated part contained internal organs in full
differentiation), regeneration proceeded normally. From their experiments, Avel and Rozier concluded that the nerve chain is essential for
the formation of a regenerative blastema during the first 3 days following transection. From the 4th day the blastema essential for the con-