190
H E N R I E T T E HERLANT-MEEWIS
are reconstructed, the segments being often very different one from
another. While regenerating, the alimentary canal reconstructs the
regions appropriate to the new segments. In Polychaetae, the stomodaeum and pharynx are of ectodermal origin, the other parts of the
alimentary canal are formed from the endoderm. In Syllis gracilis,
Baltardive (1958) demonstrated the formation of an ectodermal swelling
from which the proboscis and proventricle arise by invagination. At the
same time, the distal part of the gut in contact with the newly formed
proventricle proliferates and gives rise to the pharynx. The morphogenetic potentials of the ecto- and endoderm are thus perfectly defined.
In Oligochaetae, the digestive tract as a whole differentiates from the
endoderm, the stomodaeum alone is of ectodermal origin (Avel, 1961;
Fig. 17). During regeneration, the digestive tract shows a certain degree
of independence. Several authors, particularly Avel (1932) and Painter
(1940) working on oligochaetes and Okada (1934) on polychaetes, have
shown that its presence is not indispensable to the development of a
cephalic regeneration bud. Furthermore, experiments carried out by
Kawakami (1961) have confirmed this. When healing is not followed by
regeneration, as for instance in the absence of the nervous system, it has
been shown recently (Avel, 1961) that the oesophagus can nevertheless
proliferate by bending backwards (Fig. 18) but the epithelium remains
undifferentiated. For this differentiation to take place, it is necessary
for the ectoderm to invaginate to give a stomodaeum, which in turn
induces pharynx formation. Such an inductive influence was suggested
earlier by Kawakami (1952). The mesodermal derivatives, muscles and
the circulatory system become specialized immediately, according to the
segment in which they develop.
The origin of the nervous system has raised contradictory interpretations. According to most authors, the old nerve chain is reconstructed
from the undifferentiated cells that it contains and penetrates the regeneration bud where it then reconstructs the whole cephalic nervous
system.
However, Avel (1961) working on Eisenia foetida described the independent formation in the regeneration bud of a cephalic nervous
system which is secondarily connected to the old one (Fig. 17). Thouveny
(1958a) observed the same phenomenon during anterior regeneration in
Polydora flava. This autonomous origin had already been described by
Goldfarb (1909) and Avel (1937) in cases of cephalic regeneration in the
absence of the old nerve cord. Avel's concept, supported by elegant
graft experiments, as well as Goldfarb's observations, were not substantiated by convincing histological pictures.
This is why we have studied this problem again in two types of experiments on Eisenia foetida : the first type (unpublished work in associa-
H E N R I E T T E HERLANT-MEEWIS
are reconstructed, the segments being often very different one from
another. While regenerating, the alimentary canal reconstructs the
regions appropriate to the new segments. In Polychaetae, the stomodaeum and pharynx are of ectodermal origin, the other parts of the
alimentary canal are formed from the endoderm. In Syllis gracilis,
Baltardive (1958) demonstrated the formation of an ectodermal swelling
from which the proboscis and proventricle arise by invagination. At the
same time, the distal part of the gut in contact with the newly formed
proventricle proliferates and gives rise to the pharynx. The morphogenetic potentials of the ecto- and endoderm are thus perfectly defined.
In Oligochaetae, the digestive tract as a whole differentiates from the
endoderm, the stomodaeum alone is of ectodermal origin (Avel, 1961;
Fig. 17). During regeneration, the digestive tract shows a certain degree
of independence. Several authors, particularly Avel (1932) and Painter
(1940) working on oligochaetes and Okada (1934) on polychaetes, have
shown that its presence is not indispensable to the development of a
cephalic regeneration bud. Furthermore, experiments carried out by
Kawakami (1961) have confirmed this. When healing is not followed by
regeneration, as for instance in the absence of the nervous system, it has
been shown recently (Avel, 1961) that the oesophagus can nevertheless
proliferate by bending backwards (Fig. 18) but the epithelium remains
undifferentiated. For this differentiation to take place, it is necessary
for the ectoderm to invaginate to give a stomodaeum, which in turn
induces pharynx formation. Such an inductive influence was suggested
earlier by Kawakami (1952). The mesodermal derivatives, muscles and
the circulatory system become specialized immediately, according to the
segment in which they develop.
The origin of the nervous system has raised contradictory interpretations. According to most authors, the old nerve chain is reconstructed
from the undifferentiated cells that it contains and penetrates the regeneration bud where it then reconstructs the whole cephalic nervous
system.
However, Avel (1961) working on Eisenia foetida described the independent formation in the regeneration bud of a cephalic nervous
system which is secondarily connected to the old one (Fig. 17). Thouveny
(1958a) observed the same phenomenon during anterior regeneration in
Polydora flava. This autonomous origin had already been described by
Goldfarb (1909) and Avel (1937) in cases of cephalic regeneration in the
absence of the old nerve cord. Avel's concept, supported by elegant
graft experiments, as well as Goldfarb's observations, were not substantiated by convincing histological pictures.
This is why we have studied this problem again in two types of experiments on Eisenia foetida : the first type (unpublished work in associa-