REGENERATION IN ANNELIDS
191
tion with J . Deligne) consisted of removing the cephalic region; in the
second type several parts of the nervous system were removed (Herlant-Meewis, 1961). Our observations regarding the reconstruction of
the nervous system during cephalic regeneration confirmed those of
Hubl (1952) : the old nerve chain seems to reconstruct all of the cephalic
nervous system, from its own replacement cells.
However, a number of histological pictures remain difficult to interpret because of the complexity of the histogenetic phenomena in a
region where all organs are repaired simultaneously. This is why we have
devised the second type of experiment in which reconstruction of the
nervous system does not interfere with that of other organs. We removed
the supra- and suboesophageal ganglia by more or less total ablation of
the peri-oesophageal ring and of the dorsal and ventral anterior connectives which connect the ganglia to the body wall. We also removed
at different points in the worms one or two adjacent ganglia of the
ventral cord. In all these cases, the nervous system was completely reconstructed. As we mentioned earlier, the nerve cord regenerates from
its own elements. The supra- and suboesophageal ganglia are reconstructed from the anterior connectives : these are made up of the peripheral sensory cell axons accompanied by replacement cells. When these
connectives are cut, the axons regenerate ; they extend backwards together with the multiplying replacement cells. When the connective
reaches segment 3 its growth stops. Terminally, replacement cells form
the rudiments of the two connecting ganglia. Afterwards cells differentiate into the several types of neurons and the central neuropile reappears. These ganglia are thus formed quite independently of the
peri-oesophageal ring, and are never derived from it. Complete removal
of the ring is rendered difficult by the presence of the visceral nervous
system which forms a second ring of ganglia. I t is located between the
peri-oesophageal connective and the digestive tract and is united to
both by many small nerves. I t has been well described by Chen (1944).
Its presence is seldom taken into account. I t has numerous contact
points with the alimentary canal, and it is very likely that, during the
study of regeneration phenomena, it has been confused with the perioesophageal ring by some authors, particularly Painter and Hubl who,
following ablation of the supra-oesophageal ganglion, described its
reconstruction from gastric epithelium. The peri-oesophageal ring is
actually a connective made up of nerve fibres issuing from cells located
in other parts of the nervous system and accompanied by neurons and
replacement cells. When it is injured by the operation, it is reconstructed
from its own elements and it also contributes to the restoration of the
neighbouring visceral nervous system, but it never participates in the
regeneration of the sub- or supra-oesophageal ganglia.
191
tion with J . Deligne) consisted of removing the cephalic region; in the
second type several parts of the nervous system were removed (Herlant-Meewis, 1961). Our observations regarding the reconstruction of
the nervous system during cephalic regeneration confirmed those of
Hubl (1952) : the old nerve chain seems to reconstruct all of the cephalic
nervous system, from its own replacement cells.
However, a number of histological pictures remain difficult to interpret because of the complexity of the histogenetic phenomena in a
region where all organs are repaired simultaneously. This is why we have
devised the second type of experiment in which reconstruction of the
nervous system does not interfere with that of other organs. We removed
the supra- and suboesophageal ganglia by more or less total ablation of
the peri-oesophageal ring and of the dorsal and ventral anterior connectives which connect the ganglia to the body wall. We also removed
at different points in the worms one or two adjacent ganglia of the
ventral cord. In all these cases, the nervous system was completely reconstructed. As we mentioned earlier, the nerve cord regenerates from
its own elements. The supra- and suboesophageal ganglia are reconstructed from the anterior connectives : these are made up of the peripheral sensory cell axons accompanied by replacement cells. When these
connectives are cut, the axons regenerate ; they extend backwards together with the multiplying replacement cells. When the connective
reaches segment 3 its growth stops. Terminally, replacement cells form
the rudiments of the two connecting ganglia. Afterwards cells differentiate into the several types of neurons and the central neuropile reappears. These ganglia are thus formed quite independently of the
peri-oesophageal ring, and are never derived from it. Complete removal
of the ring is rendered difficult by the presence of the visceral nervous
system which forms a second ring of ganglia. I t is located between the
peri-oesophageal connective and the digestive tract and is united to
both by many small nerves. I t has been well described by Chen (1944).
Its presence is seldom taken into account. I t has numerous contact
points with the alimentary canal, and it is very likely that, during the
study of regeneration phenomena, it has been confused with the perioesophageal ring by some authors, particularly Painter and Hubl who,
following ablation of the supra-oesophageal ganglion, described its
reconstruction from gastric epithelium. The peri-oesophageal ring is
actually a connective made up of nerve fibres issuing from cells located
in other parts of the nervous system and accompanied by neurons and
replacement cells. When it is injured by the operation, it is reconstructed
from its own elements and it also contributes to the restoration of the
neighbouring visceral nervous system, but it never participates in the
regeneration of the sub- or supra-oesophageal ganglia.