MORPHOGENESIS OF FRESH-WATER PLANARIA
315
In this case, the brain was not the inducer of the regeneration of the
pharynx since the prepharyngeal region itself was an inducer even if the
cerebral ganglia were removed.
Sengel (1951a and b) studied the normal regeneration of the pharynx
of Dugesia lugubris. The pharynx, excised with its sheath, regenerated
normally in the absence of the head and the prepharyngeal region.
Therefore, the differentiation of the pharynx was not induced directly
by the head or the prepharyngeal region. The latter itself must possess
the ability to regenerate a pharynx.
Sengel (1953) repeated the experiments of Santos. A graft of the head
of Dugesia lugubris after 3 to 4 weeks induced one or two supplementary pharynges in the postpharyngeal region of the host. If the head
graft and the induced pharynx were removed, the latter regenerated in
the absence of any graft tissue (Fig. 6). It was not the pharynx which
had been induced by the graft but rather a pharyngeal zone which had
the capacity to autodifferentiate a pharynx. Sengel (1953) concluded
that the regeneration of a pharynx was brought about by a double
phenomenon of induction. The anterior regions, when grafted to the
caudal region, caused a 'pharyngization' of the host tissue. The dominant
region induced a dominated zone. The latter, once determined, brought
about the regeneration of the pharynx.
Chandebois (1957, 1960a) does not think that the regeneration of the
pharynx of the marine planarian Procerodes lobata or of the fresh-water
planarian Planaria subtentaculata is caused by an induction by the
anterior region. He considers that in the absence of a caudal bud,
regeneration would not be possible but would depend exclusively on the
regulation of the anterio-posterior system of the planarian.
The experiments of Wolff, Sengel, and Sengel (1958) and of C. Sengel
(1959a) failed to confirm this point of view. These authors excised the
pharyngeal region and studied regeneration of the pharynx from prepharyngeal and postpharyngeal fragments in Dugesia lugubris and
Polycelis nigra. The prepharyngeal fragment differentiated a pharynx
inside the old tissues. The postpharyngeal fragment regenerated the
pharynx in the blastema at the same time as the brain and the eyes.
If the regeneration bud of each of the two fragments was excised every
day, the pharynx differentiated from the anterior fragment, but never
from the posterior fragment. It was also possible to delay the formation
of a regeneration blastema by X-irradiation of a zone near the cut. A
pharynx regenerated from the anterior fragment before the appearance
of a posterior regeneration blastema. In the posterior fragment, the
pharynx appeared when a regeneration blastema had built up a head
with eyes. Thus, the postpharyngeal region could not induce the
regeneration of a pharynx, which could differentiate only under the
315
In this case, the brain was not the inducer of the regeneration of the
pharynx since the prepharyngeal region itself was an inducer even if the
cerebral ganglia were removed.
Sengel (1951a and b) studied the normal regeneration of the pharynx
of Dugesia lugubris. The pharynx, excised with its sheath, regenerated
normally in the absence of the head and the prepharyngeal region.
Therefore, the differentiation of the pharynx was not induced directly
by the head or the prepharyngeal region. The latter itself must possess
the ability to regenerate a pharynx.
Sengel (1953) repeated the experiments of Santos. A graft of the head
of Dugesia lugubris after 3 to 4 weeks induced one or two supplementary pharynges in the postpharyngeal region of the host. If the head
graft and the induced pharynx were removed, the latter regenerated in
the absence of any graft tissue (Fig. 6). It was not the pharynx which
had been induced by the graft but rather a pharyngeal zone which had
the capacity to autodifferentiate a pharynx. Sengel (1953) concluded
that the regeneration of a pharynx was brought about by a double
phenomenon of induction. The anterior regions, when grafted to the
caudal region, caused a 'pharyngization' of the host tissue. The dominant
region induced a dominated zone. The latter, once determined, brought
about the regeneration of the pharynx.
Chandebois (1957, 1960a) does not think that the regeneration of the
pharynx of the marine planarian Procerodes lobata or of the fresh-water
planarian Planaria subtentaculata is caused by an induction by the
anterior region. He considers that in the absence of a caudal bud,
regeneration would not be possible but would depend exclusively on the
regulation of the anterio-posterior system of the planarian.
The experiments of Wolff, Sengel, and Sengel (1958) and of C. Sengel
(1959a) failed to confirm this point of view. These authors excised the
pharyngeal region and studied regeneration of the pharynx from prepharyngeal and postpharyngeal fragments in Dugesia lugubris and
Polycelis nigra. The prepharyngeal fragment differentiated a pharynx
inside the old tissues. The postpharyngeal fragment regenerated the
pharynx in the blastema at the same time as the brain and the eyes.
If the regeneration bud of each of the two fragments was excised every
day, the pharynx differentiated from the anterior fragment, but never
from the posterior fragment. It was also possible to delay the formation
of a regeneration blastema by X-irradiation of a zone near the cut. A
pharynx regenerated from the anterior fragment before the appearance
of a posterior regeneration blastema. In the posterior fragment, the
pharynx appeared when a regeneration blastema had built up a head
with eyes. Thus, the postpharyngeal region could not induce the
regeneration of a pharynx, which could differentiate only under the