174
PAUL BRIEN
they histoblasts of mesenchyme or mesoblasts, with their histological
orientation determined, which have become migrators, the presence of
which would be indispensable in the zone of regeneration in order to form
the blastema of regeneration? It must be recalled that in the regeneration
of serpulids and in the budding of Salmacina, Cresp (1964) observed no
migration of mesodermal elements and that "the idea of pluripotent or
totipotent neoblasts, colonizing various layers by emigrating, must be
categorically denied for the two species studied."
Migrating neoblasts do exist, however. Planarians provide a notable
example of them, pointed out some time ago by Curtis (1902). These
neoblasts in planarians are part of the mesenchyme, and are distributed
throughout the body. The cells are elongate, ovoid, spindle-shaped, with
a large nucleolus in a pale nucleus, and with cytoplasm rich in ribonucleic
acid (Clement-Noel, 1944; Lender and Gabriel, 1960, 1961). These cells
move to the point of the cut to form the blastema of regeneration to
replace the organs removed (Dubois, 1949). Lender and Gabriel (1960)
showed that their distribution is changed after an amputation: an inflow
occurs in the region of regeneration when their density decreases everywhere else. The neoblasts, like all embryonic cells, are very sensitive to
X-rays. After irradiation they are killed; no regeneration occurs. Wolff
and Dubois (1948) have performed the following experiment. They irradiated just one area of the body: one-tenth or three-tenth in the anterior region of the planarian. After irradiation the head was cut.
Regeneration does take place, but with a certain delay in comparison to
the controls. The delay is greater as the zone irradiated is longer. When
regeneration of the head is accomplished in 7 days in normal planarians,
it will take 15 or 20 days if the anterior one-tenth of the body is irradiated, 27 or 33 days if the anterior three-tenth has been affected. This
delay corresponds to the time necessary for intact neoblasts to cross the
irradiated area. If, in a planarian totally irradiated and thus without
neoblasts, a fragment of a healthy planarian is implanted, the operated
animal survives. Sectioning of the head is followed by regeneration, as a
result of an influx of neoblasts from the graft which, after having invaded
the irradiated planarian, form a blastema of regeneration (Dubois, 1949).
Lender and Gabriel (1965) have furnished a new demonstration. Planarians (Dugesia gonocephala)
are labeled with tritiated uridine for 24 to 48
hours. The prepharyngeal region is removed, then implanted in place of
the prepharyngeal region of a planarian previously irradiated. The
planarians thus irradiated and grafted are decapitated. A blastema of
PAUL BRIEN
they histoblasts of mesenchyme or mesoblasts, with their histological
orientation determined, which have become migrators, the presence of
which would be indispensable in the zone of regeneration in order to form
the blastema of regeneration? It must be recalled that in the regeneration
of serpulids and in the budding of Salmacina, Cresp (1964) observed no
migration of mesodermal elements and that "the idea of pluripotent or
totipotent neoblasts, colonizing various layers by emigrating, must be
categorically denied for the two species studied."
Migrating neoblasts do exist, however. Planarians provide a notable
example of them, pointed out some time ago by Curtis (1902). These
neoblasts in planarians are part of the mesenchyme, and are distributed
throughout the body. The cells are elongate, ovoid, spindle-shaped, with
a large nucleolus in a pale nucleus, and with cytoplasm rich in ribonucleic
acid (Clement-Noel, 1944; Lender and Gabriel, 1960, 1961). These cells
move to the point of the cut to form the blastema of regeneration to
replace the organs removed (Dubois, 1949). Lender and Gabriel (1960)
showed that their distribution is changed after an amputation: an inflow
occurs in the region of regeneration when their density decreases everywhere else. The neoblasts, like all embryonic cells, are very sensitive to
X-rays. After irradiation they are killed; no regeneration occurs. Wolff
and Dubois (1948) have performed the following experiment. They irradiated just one area of the body: one-tenth or three-tenth in the anterior region of the planarian. After irradiation the head was cut.
Regeneration does take place, but with a certain delay in comparison to
the controls. The delay is greater as the zone irradiated is longer. When
regeneration of the head is accomplished in 7 days in normal planarians,
it will take 15 or 20 days if the anterior one-tenth of the body is irradiated, 27 or 33 days if the anterior three-tenth has been affected. This
delay corresponds to the time necessary for intact neoblasts to cross the
irradiated area. If, in a planarian totally irradiated and thus without
neoblasts, a fragment of a healthy planarian is implanted, the operated
animal survives. Sectioning of the head is followed by regeneration, as a
result of an influx of neoblasts from the graft which, after having invaded
the irradiated planarian, form a blastema of regeneration (Dubois, 1949).
Lender and Gabriel (1965) have furnished a new demonstration. Planarians (Dugesia gonocephala)
are labeled with tritiated uridine for 24 to 48
hours. The prepharyngeal region is removed, then implanted in place of
the prepharyngeal region of a planarian previously irradiated. The
planarians thus irradiated and grafted are decapitated. A blastema of