MORPHOGENESIS OF FRESH-WATER PLANARIA
313
If a head blastema and a tail blastema were brought together, the
aggregate elongated and took the form of a small planarian. The head
blastema regenerated the brain and the eyes, the pharynx differentiated
in the middle region. These experiments showed clearly that on the
second or third day, therefore, before the appearance of differentiated
tissues, the blastema is already determined. Reconstitution of a
complete planarian in vitro is possible only by association of a head
blastema and a tail blastema. The determination of the blastema takes
place very rapidly, in the first two days after amputation. But at this
moment the regeneration bud is so small that it would be hard to isolate.
The conditions and factors which determine the quality of the
regenerated organism are still poorly understood. In the most general
context, they involve the explanation of polarity in the planarians.
We shall not deal here with the older theories which are discussed elsewhere (Abeloos, 1932; Needham, 1952; Br0ndsted, 1955). The study of
the regeneration of the brain gives us occasion to deal with the recent
explanations.
B. Organ Differentiation in Regeneration
During the first phases of regeneration, the blastema formed by the
neoblasts continues to grow. But morphogenesis begins very rapidly;
the organs become differentiated.
The analysis of the regeneration of the principal organs will show
from which elements they are built up and which factors play a role in
their differentiation.
1. Regeneration of the Pharynx
A pharynx may regenerate either from the blastema or from the old
tissues of the planarian. In every instance an accumulation of neoblasts
is observed at the start. It may be noted that Gaszo (1958) postulates
that the regeneration of the pharynx of Dendrocoelum lacteum occurs
primarily from the conjunctive cells and the muscle cells of the copulatory apparatus which are dedifferentiated under the influence of the
traumatism. Unfortunately the author does not offer any proof of this
dedifferentiation.
A cavity appears among the cells of the blastema. It is covered by the
regenerated sheath of the pharynx. A bud begins to form in the space
which has thus been defined. It grows and differentiates into a pharynx
(Dresden, 1940; Asperen 1946).
During a study of the regeneration of the pharynx, Santos (1929,
1931) for the first time demonstrated the participation of an inductive
mechanism in the regeneration of fresh-water planarians. A graft of the
head of a Planaria dorotocephala or a Planaria maculata (Dugesia
313
If a head blastema and a tail blastema were brought together, the
aggregate elongated and took the form of a small planarian. The head
blastema regenerated the brain and the eyes, the pharynx differentiated
in the middle region. These experiments showed clearly that on the
second or third day, therefore, before the appearance of differentiated
tissues, the blastema is already determined. Reconstitution of a
complete planarian in vitro is possible only by association of a head
blastema and a tail blastema. The determination of the blastema takes
place very rapidly, in the first two days after amputation. But at this
moment the regeneration bud is so small that it would be hard to isolate.
The conditions and factors which determine the quality of the
regenerated organism are still poorly understood. In the most general
context, they involve the explanation of polarity in the planarians.
We shall not deal here with the older theories which are discussed elsewhere (Abeloos, 1932; Needham, 1952; Br0ndsted, 1955). The study of
the regeneration of the brain gives us occasion to deal with the recent
explanations.
B. Organ Differentiation in Regeneration
During the first phases of regeneration, the blastema formed by the
neoblasts continues to grow. But morphogenesis begins very rapidly;
the organs become differentiated.
The analysis of the regeneration of the principal organs will show
from which elements they are built up and which factors play a role in
their differentiation.
1. Regeneration of the Pharynx
A pharynx may regenerate either from the blastema or from the old
tissues of the planarian. In every instance an accumulation of neoblasts
is observed at the start. It may be noted that Gaszo (1958) postulates
that the regeneration of the pharynx of Dendrocoelum lacteum occurs
primarily from the conjunctive cells and the muscle cells of the copulatory apparatus which are dedifferentiated under the influence of the
traumatism. Unfortunately the author does not offer any proof of this
dedifferentiation.
A cavity appears among the cells of the blastema. It is covered by the
regenerated sheath of the pharynx. A bud begins to form in the space
which has thus been defined. It grows and differentiates into a pharynx
(Dresden, 1940; Asperen 1946).
During a study of the regeneration of the pharynx, Santos (1929,
1931) for the first time demonstrated the participation of an inductive
mechanism in the regeneration of fresh-water planarians. A graft of the
head of a Planaria dorotocephala or a Planaria maculata (Dugesia