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H E N R I E T T E H E R L A N T - M E E W I S
all the polychaetes. On the contrary, most papers dealing with these
annelids emphasize the independent reconstruction of the three layers
from their own regenerative elements. Sicard-Bruslé (1957) considered
that this was so for posterior regeneration in N. massiliensis, Cresp
(1957) did the same for cephalic regeneration in Salmacina incrustans,
and so did Thouveny (1958b, 1959) for caudal regeneration in Polydora
flava, Durand (1959) in Dasychone lucullana, Boilly (1962b) in Syllis
arnica and Clark and Clark (1962) in Nephtys. In all these cases, although
cell penetration from one layer into another was not observed during
blastema elaboration, it is a fact that close physiological relations do
exist between these three layers. Thus, in most oligochaetes, mesodermal elements change form, migrate and multiply, starting on the
ventral side close to the nervous system. These mesodermal elements of
the blastema are the first to reach their position and induce transformations in the adjacent ectoderm. In some oligochaetes, such as Aeolosoma,
forms in which the nervous system is entirely enclosed in the ectoderm,
the ectodermal cells adjacent to the nervous system change form before
the mesoderm has been modified (Fig. 2), and it is the ectodermal
blastocytes which seem to induce parietopleural changes (Fig. 3). The
same is true of many polychaetes whose nervous system is in close
connexion with the ectoderm. The same feature was observed in
Salmacina incrustans (Cresp, 1957). Without regeneration, activation
of the ectoderm does not involve that of the adjacent mesoderm. Thus,
in Dasychone lucullana, after transection in the anterior thoracic region
at a level where there is never a caudal regeneration, Durand (1959)
observed dedifferentiation of epidermal cells adjacent to the nervous
system, but the mesoderm remained inactive and development stopped
at the wound-healing stage. The lack of caudal and cephalic regenerative
power seems to be linked with the failure of a mesodermal tissue reaction (La Greca, 1953a, b).
B. Biochemical Factors
In annelids, few experiments have been made to establish the nature
of the role played by the nervous system in the course of wound-healing
and blastema formation. Avel (1961) has recently shown that limb regeneration in urodele amphibians and cephalic reconstruction in
Lumbricidae are comparable phenomena. In amphibians, according to
Karczmar (1946), degenerating nerve fibres can secrete a substance
which stimulates dedifferentiation of adjacent tissue and Schwann cell
multiplication, observed by Abercrombie and Johnson (1942), which is
a prerequisite for nerve fibre regeneration.
In oligochaetes, it is possible that similar substances, released
immediately after cutting through the nerve cord participate in neo-
H E N R I E T T E H E R L A N T - M E E W I S
all the polychaetes. On the contrary, most papers dealing with these
annelids emphasize the independent reconstruction of the three layers
from their own regenerative elements. Sicard-Bruslé (1957) considered
that this was so for posterior regeneration in N. massiliensis, Cresp
(1957) did the same for cephalic regeneration in Salmacina incrustans,
and so did Thouveny (1958b, 1959) for caudal regeneration in Polydora
flava, Durand (1959) in Dasychone lucullana, Boilly (1962b) in Syllis
arnica and Clark and Clark (1962) in Nephtys. In all these cases, although
cell penetration from one layer into another was not observed during
blastema elaboration, it is a fact that close physiological relations do
exist between these three layers. Thus, in most oligochaetes, mesodermal elements change form, migrate and multiply, starting on the
ventral side close to the nervous system. These mesodermal elements of
the blastema are the first to reach their position and induce transformations in the adjacent ectoderm. In some oligochaetes, such as Aeolosoma,
forms in which the nervous system is entirely enclosed in the ectoderm,
the ectodermal cells adjacent to the nervous system change form before
the mesoderm has been modified (Fig. 2), and it is the ectodermal
blastocytes which seem to induce parietopleural changes (Fig. 3). The
same is true of many polychaetes whose nervous system is in close
connexion with the ectoderm. The same feature was observed in
Salmacina incrustans (Cresp, 1957). Without regeneration, activation
of the ectoderm does not involve that of the adjacent mesoderm. Thus,
in Dasychone lucullana, after transection in the anterior thoracic region
at a level where there is never a caudal regeneration, Durand (1959)
observed dedifferentiation of epidermal cells adjacent to the nervous
system, but the mesoderm remained inactive and development stopped
at the wound-healing stage. The lack of caudal and cephalic regenerative
power seems to be linked with the failure of a mesodermal tissue reaction (La Greca, 1953a, b).
B. Biochemical Factors
In annelids, few experiments have been made to establish the nature
of the role played by the nervous system in the course of wound-healing
and blastema formation. Avel (1961) has recently shown that limb regeneration in urodele amphibians and cephalic reconstruction in
Lumbricidae are comparable phenomena. In amphibians, according to
Karczmar (1946), degenerating nerve fibres can secrete a substance
which stimulates dedifferentiation of adjacent tissue and Schwann cell
multiplication, observed by Abercrombie and Johnson (1942), which is
a prerequisite for nerve fibre regeneration.
In oligochaetes, it is possible that similar substances, released
immediately after cutting through the nerve cord participate in neo-