REGENERATION IN ANNELIDS
177
generation occurred and he inferred from this that the nerve chain did
not play an essential part in regeneration of the head. In 1933, Kropp
used the same method and obtained similar results with Helodrilus
caliginosus. This conclusion was confirmed by Avel (1932, 1938, 1947)
and Bailey (1939), from experiments involving the bending backwards
of the nerve cord as described and shown in Fig. 15B (Bailey) and Fig.
16 (Avel). In a few instances, the regenerated head was smaller than
usual and often abnormal in other ways. In Eisenia foetida typica Avel
obtained regeneration in about 20% of cases ; he indicated, however,
that although regeneration may be organically complete, the process is
much slower than in controls and organ differentiation is incomplete in
most cases ; this applies especially to the nervous system which may not
be functional even after 2 months, whereas function is restored in 1
month in control specimens. Recently Avel (1961) showed that if the
identical operations are performed in another variety of the same species,
Eisenia foetida unicolor, regeneration never occurs. In Figs. 17 and 18 a
comparison is made of regenerative processes in this species, in the
absence and presence of the nervous system at the point of healing. In
its absence, a certain amount of proliferation occurs: in particular the
oesophagus can grow by bending backwards towards the nervous system
but, if the stomodaeum is absent, it does not differentiate into a pharynx.
There is no external regeneration. Various results have also been obtained during posterior regeneration: thus, according to Holmes (1931),
caudal regeneration may or may not occur in Nereis ; these are, however,
often small, asymmetrical and poorly segmented.
We have recently reconsidered this problem (in collaboration with J.
Deligne, unpublished) in Eisenia foetida. The worms were amputated at
the fourth segment and a ventral flap was cut into the adjacent segments
to reach the nerve cord. The nerves were then transected prior to freeing
the nerve cord by cauterization or severance at segment 7 as shown in
Fig. 19. The results were of two types : either cephalic regeneration was
normal (but delayed or abnormal in respect of the size of the regenerated
part) or there was a complete lack of head reconstruction. Histological
examination of the regenerated parts at different developmental stages
showed why these apparently contradictory results were obtained.
When the head of Eisenia is severed at segment 4, multiplication of the
regenerating cells of the nerve chain is immediate and growth of the
proximal stump of the nerve is also rapidly triggered off. Histogenesis
of the nervous system takes place at the same time as the healing
blastema develops so that the growing nerve fibres penetrate the new
tissue. Later on, the reconstructed old nerve cord regenerates the cephalic
nervous system. When removal of the anterior segments is accompanied
by extraction of the nerve cord in two or three of the following segments,
177
generation occurred and he inferred from this that the nerve chain did
not play an essential part in regeneration of the head. In 1933, Kropp
used the same method and obtained similar results with Helodrilus
caliginosus. This conclusion was confirmed by Avel (1932, 1938, 1947)
and Bailey (1939), from experiments involving the bending backwards
of the nerve cord as described and shown in Fig. 15B (Bailey) and Fig.
16 (Avel). In a few instances, the regenerated head was smaller than
usual and often abnormal in other ways. In Eisenia foetida typica Avel
obtained regeneration in about 20% of cases ; he indicated, however,
that although regeneration may be organically complete, the process is
much slower than in controls and organ differentiation is incomplete in
most cases ; this applies especially to the nervous system which may not
be functional even after 2 months, whereas function is restored in 1
month in control specimens. Recently Avel (1961) showed that if the
identical operations are performed in another variety of the same species,
Eisenia foetida unicolor, regeneration never occurs. In Figs. 17 and 18 a
comparison is made of regenerative processes in this species, in the
absence and presence of the nervous system at the point of healing. In
its absence, a certain amount of proliferation occurs: in particular the
oesophagus can grow by bending backwards towards the nervous system
but, if the stomodaeum is absent, it does not differentiate into a pharynx.
There is no external regeneration. Various results have also been obtained during posterior regeneration: thus, according to Holmes (1931),
caudal regeneration may or may not occur in Nereis ; these are, however,
often small, asymmetrical and poorly segmented.
We have recently reconsidered this problem (in collaboration with J.
Deligne, unpublished) in Eisenia foetida. The worms were amputated at
the fourth segment and a ventral flap was cut into the adjacent segments
to reach the nerve cord. The nerves were then transected prior to freeing
the nerve cord by cauterization or severance at segment 7 as shown in
Fig. 19. The results were of two types : either cephalic regeneration was
normal (but delayed or abnormal in respect of the size of the regenerated
part) or there was a complete lack of head reconstruction. Histological
examination of the regenerated parts at different developmental stages
showed why these apparently contradictory results were obtained.
When the head of Eisenia is severed at segment 4, multiplication of the
regenerating cells of the nerve chain is immediate and growth of the
proximal stump of the nerve is also rapidly triggered off. Histogenesis
of the nervous system takes place at the same time as the healing
blastema develops so that the growing nerve fibres penetrate the new
tissue. Later on, the reconstructed old nerve cord regenerates the cephalic
nervous system. When removal of the anterior segments is accompanied
by extraction of the nerve cord in two or three of the following segments,