166
H E N R I E T T E HERLANT-MEEWIS
the mesodermal regeneration cells derived from neoblasts do not
penetrate between these walls immediately but do so later. Formation of
the mesoderm is illustrated in Fig. 9. These mesodermal elements never
participate in the reconstruction of the nervous system nor in the
renovation of the ecto- and endoderm, which remain thin and little
differentiated. Eventually the lack of harmony between the development of a sound mesoderm and the deficient epithelial walls leads to a
breakdown of the regeneration bud and its gradual disappearance.
Stephan-Dubois concluded that in the oligochaetes she studied, regenerative elements derived from the mesoderm were incapable of
participating in the regeneration of the ecto- and endodermal walls.
In Lumbricillus lineatus we have shown (Herlant-Meewis, 1946) that
migrating neoblasts are absent. Stephan-Dubois corroborates this
observation in another enchytraeid, Enchytraeus albidus. Parietopleural
cells in an irradiated segment change in situ into mesodermal blastocytes but this cell multiplication is strongly diminished and differentiation into mesodermal elements is not important. No activated mesodermal elements appear in the non-irradiated anterior segments. The
endo- and ectoderm remain thin, replacement cells that have been
destroyed are not replaced, and the development of organs derived
from the ectoderm, such as setae, remains uncertain. The nervous system does not develop in the regeneration bud and the latter soon begins
to disintegrate. Here, mesodermal regeneration cells are even less
capable than in the other two species of participating in the reconstruction of ecto- and endoderm.
In 1958, Stephan-Dubois used the local irradiation technique on a
polychaete, Nereis diversicolor, another species lacking migrating neoblasts. She showed that regenerative cells in this species are acidophilic
amoebocytes which enlarge, become basophilic and slide over the surface of the cicatricial plug forming an epidermal layer and secreting a
cuticle. Identical cells penetrate the endoderm where they proliferate
and gather in small clusters destined for regeneration of the wall of the
digestive tract. Similar coelomocytes from neighbouring segments also
participate in the regeneration of mesodermal organs and at least in
part of the musculature. They could also be the origin of nerve cells in
the regeneration bud. According to Stephan-Dubois, these amoebocytes
would therefore be totipotent. If four segments anterior to the point of
transection are X-irradiated, development of the regeneration bud is
abnormal for 4 weeks, becoming normal afterwards. Here, as in the case
of neoblasts, healing takes place and amoebocytes derived from
irradiated segments, migrate towards the healing zone and penetrate
the ectoderm and endoderm. In the ectoderm, they become very large
due to activation and they look like neoblasts. They are, however,
H E N R I E T T E HERLANT-MEEWIS
the mesodermal regeneration cells derived from neoblasts do not
penetrate between these walls immediately but do so later. Formation of
the mesoderm is illustrated in Fig. 9. These mesodermal elements never
participate in the reconstruction of the nervous system nor in the
renovation of the ecto- and endoderm, which remain thin and little
differentiated. Eventually the lack of harmony between the development of a sound mesoderm and the deficient epithelial walls leads to a
breakdown of the regeneration bud and its gradual disappearance.
Stephan-Dubois concluded that in the oligochaetes she studied, regenerative elements derived from the mesoderm were incapable of
participating in the regeneration of the ecto- and endodermal walls.
In Lumbricillus lineatus we have shown (Herlant-Meewis, 1946) that
migrating neoblasts are absent. Stephan-Dubois corroborates this
observation in another enchytraeid, Enchytraeus albidus. Parietopleural
cells in an irradiated segment change in situ into mesodermal blastocytes but this cell multiplication is strongly diminished and differentiation into mesodermal elements is not important. No activated mesodermal elements appear in the non-irradiated anterior segments. The
endo- and ectoderm remain thin, replacement cells that have been
destroyed are not replaced, and the development of organs derived
from the ectoderm, such as setae, remains uncertain. The nervous system does not develop in the regeneration bud and the latter soon begins
to disintegrate. Here, mesodermal regeneration cells are even less
capable than in the other two species of participating in the reconstruction of ecto- and endoderm.
In 1958, Stephan-Dubois used the local irradiation technique on a
polychaete, Nereis diversicolor, another species lacking migrating neoblasts. She showed that regenerative cells in this species are acidophilic
amoebocytes which enlarge, become basophilic and slide over the surface of the cicatricial plug forming an epidermal layer and secreting a
cuticle. Identical cells penetrate the endoderm where they proliferate
and gather in small clusters destined for regeneration of the wall of the
digestive tract. Similar coelomocytes from neighbouring segments also
participate in the regeneration of mesodermal organs and at least in
part of the musculature. They could also be the origin of nerve cells in
the regeneration bud. According to Stephan-Dubois, these amoebocytes
would therefore be totipotent. If four segments anterior to the point of
transection are X-irradiated, development of the regeneration bud is
abnormal for 4 weeks, becoming normal afterwards. Here, as in the case
of neoblasts, healing takes place and amoebocytes derived from
irradiated segments, migrate towards the healing zone and penetrate
the ectoderm and endoderm. In the ectoderm, they become very large
due to activation and they look like neoblasts. They are, however,