156
H E N R I E T T E HERLANT-MEEWIS
The initiation and cessation of these phenomena are linked with the
biochemical evolution of certain cells. The unfolding of histogenetic
processes also depends on internal and external factors, and it is important to take them into account. Of the many factors to be considered,
we shall place particular emphasis on the role played by the nervous
system during the different regenerative stages, especially neurohormonal phenomena which have recently been introduced into the
study of annelids.
II. Blastema Formation
A. Regenerative Elements
In 1952, Needham analysed the initiating factors of regeneration.
One of the most controversial problems in the field of annelid regeneration is that of the origin of the regenerative elements, their activation,
transformation and migration during wound-healing, and the elaboration of the regeneration blastema. Since the end of last century, several
authors have studied these processes in Oligochaetae and Polychaetae.
This work is covered by an extensive bibliography (Herlant-Meewis,
1947). Stolte (1955) expanded this to take in work published until 1950,
especially with regard to the Oligochaetae. In this chapter we shall
consider more recent work.
Annelids are primitive organisms and owe their high regenerative
power to the incomplete differentiation of their tissues. Either the cells
remain undifferentiated, or certain cell components remain capable of
easy dedifferentiation. These cells are found in the three layers—ectoderm, mesoderm and endoderm—whose repair they guarantee, either
by taking part in the reconstruction of their own particular layer and
its derivatives, or by exhibiting totipotency so that any type of blastema
component can be elaborated. The latter explanation is the older view.
Several authors at the end of last century reached the conclusion that
organs of the regeneration bud were derived solely from ectodermal
cells; others thought that only mesodermal blastocytes were involved.
Both hypotheses still have their supporters, but generally speaking
most recent authors acknowledge a certain independence of the layers.
The embryonic tissue reserve can be viewed as either the neoblast or
the 'replacement cell'—the 'Ersatszell' of the German authors. A neoblast is a large element characterized by a vesicular, voluminous
nucleus with a prominent nucleolus: they are found particularly in
oligochaetes reproducing asexually through fragmentation (Lumbriculus) or fissiparity (limicolous oligochaetes). They are localized in the
mesoderm along the nerve cord ; there are one or two in each segment.
Figure 1 shows their appearance in two different regions of Lumbriculus
variegatus. Replacement cells are small elements enclosed at the base of
H E N R I E T T E HERLANT-MEEWIS
The initiation and cessation of these phenomena are linked with the
biochemical evolution of certain cells. The unfolding of histogenetic
processes also depends on internal and external factors, and it is important to take them into account. Of the many factors to be considered,
we shall place particular emphasis on the role played by the nervous
system during the different regenerative stages, especially neurohormonal phenomena which have recently been introduced into the
study of annelids.
II. Blastema Formation
A. Regenerative Elements
In 1952, Needham analysed the initiating factors of regeneration.
One of the most controversial problems in the field of annelid regeneration is that of the origin of the regenerative elements, their activation,
transformation and migration during wound-healing, and the elaboration of the regeneration blastema. Since the end of last century, several
authors have studied these processes in Oligochaetae and Polychaetae.
This work is covered by an extensive bibliography (Herlant-Meewis,
1947). Stolte (1955) expanded this to take in work published until 1950,
especially with regard to the Oligochaetae. In this chapter we shall
consider more recent work.
Annelids are primitive organisms and owe their high regenerative
power to the incomplete differentiation of their tissues. Either the cells
remain undifferentiated, or certain cell components remain capable of
easy dedifferentiation. These cells are found in the three layers—ectoderm, mesoderm and endoderm—whose repair they guarantee, either
by taking part in the reconstruction of their own particular layer and
its derivatives, or by exhibiting totipotency so that any type of blastema
component can be elaborated. The latter explanation is the older view.
Several authors at the end of last century reached the conclusion that
organs of the regeneration bud were derived solely from ectodermal
cells; others thought that only mesodermal blastocytes were involved.
Both hypotheses still have their supporters, but generally speaking
most recent authors acknowledge a certain independence of the layers.
The embryonic tissue reserve can be viewed as either the neoblast or
the 'replacement cell'—the 'Ersatszell' of the German authors. A neoblast is a large element characterized by a vesicular, voluminous
nucleus with a prominent nucleolus: they are found particularly in
oligochaetes reproducing asexually through fragmentation (Lumbriculus) or fissiparity (limicolous oligochaetes). They are localized in the
mesoderm along the nerve cord ; there are one or two in each segment.
Figure 1 shows their appearance in two different regions of Lumbriculus
variegatus. Replacement cells are small elements enclosed at the base of