BLASTOGENESIS AND MORPHOGENESIS
173
the ascidiozooid. In the case of Aplidium zostericola, however, it does not
play a role in the formation of the digestive loop. In this task it is replaced by the two trunks of the former digestive loop (Figs. 13 and 14),
but these, in the course of organogenesis of the bud, undergo a rejuvenation which takes them back to the histological condition of newly formed
tissues (Brien, 1924).
Morphallaxis is a reshaping, a rejuvenation of older tissues, under the
inductive influence of young regenerating tissues, and, in correlation with
them, a dedifferentiation without modification of their initial histological
orientation.
Morphallaxis is thus the expression of reversibility in histological
specificity, always possible within a threshold of differentiation, but beyond which it is irreversible, exactly as in the case of sex reversal of the
gonocytes in vertebrates, invertebrates, and hydras.
Morphallaxis of tissues which contribute to organogenesis of younger
tissues appears like the retroactive effect of the induction which they
themselves exerted on the regenerating cells in the orientation of processes
of epimorphosis. They, in turn, undergo induction by the young tissues in
organogenesis, giving an example of the effects of actions and reactions,
which constitute the autoregulation of all ontogenesis.
B. Histoblasts and Neoblasts
Some authors have pointed out that mesodermal histoblasts are capable of shifting position to form a blastema of regeneration. HerlantMeewis (1946) describes in Nais elinguis histoblasts in the coelomic wall,
quiescent in resting segments, but which, in segments close to a cut and
at the time of regeneration, enlarge, become very basophilic, rich in
ribonucleic acid, and capable of shifting position in order to take part in
the blastema of regeneration at the point of wound healing.
Dubois-Stephan (1954) has shown that this signal is sensed in two or
three segments (Tubifex)
or even in the nine or ten segments
(Lumbriculus) which precede the cut. If these segments, susceptible to being thus
activated, are treated with X-rays before injury, their mesoblastic neoblasts become necrotic. A second signal for neoblasts is produced in the
5 days following the operation. This time they come from segments much
farther away, and up to that time, intact. Crossing the irradiated segments, they reach the area of amputation in 2 or 3 days. It would be
the same in the polychaete, Nereis diversicolor (Dubois-Stephen, 1958).
In these cases, is it a question of true neoblasts, that is, of polyvalent
cells, capable of many types of histological differentiation? Or rather, are
173
the ascidiozooid. In the case of Aplidium zostericola, however, it does not
play a role in the formation of the digestive loop. In this task it is replaced by the two trunks of the former digestive loop (Figs. 13 and 14),
but these, in the course of organogenesis of the bud, undergo a rejuvenation which takes them back to the histological condition of newly formed
tissues (Brien, 1924).
Morphallaxis is a reshaping, a rejuvenation of older tissues, under the
inductive influence of young regenerating tissues, and, in correlation with
them, a dedifferentiation without modification of their initial histological
orientation.
Morphallaxis is thus the expression of reversibility in histological
specificity, always possible within a threshold of differentiation, but beyond which it is irreversible, exactly as in the case of sex reversal of the
gonocytes in vertebrates, invertebrates, and hydras.
Morphallaxis of tissues which contribute to organogenesis of younger
tissues appears like the retroactive effect of the induction which they
themselves exerted on the regenerating cells in the orientation of processes
of epimorphosis. They, in turn, undergo induction by the young tissues in
organogenesis, giving an example of the effects of actions and reactions,
which constitute the autoregulation of all ontogenesis.
B. Histoblasts and Neoblasts
Some authors have pointed out that mesodermal histoblasts are capable of shifting position to form a blastema of regeneration. HerlantMeewis (1946) describes in Nais elinguis histoblasts in the coelomic wall,
quiescent in resting segments, but which, in segments close to a cut and
at the time of regeneration, enlarge, become very basophilic, rich in
ribonucleic acid, and capable of shifting position in order to take part in
the blastema of regeneration at the point of wound healing.
Dubois-Stephan (1954) has shown that this signal is sensed in two or
three segments (Tubifex)
or even in the nine or ten segments
(Lumbriculus) which precede the cut. If these segments, susceptible to being thus
activated, are treated with X-rays before injury, their mesoblastic neoblasts become necrotic. A second signal for neoblasts is produced in the
5 days following the operation. This time they come from segments much
farther away, and up to that time, intact. Crossing the irradiated segments, they reach the area of amputation in 2 or 3 days. It would be
the same in the polychaete, Nereis diversicolor (Dubois-Stephen, 1958).
In these cases, is it a question of true neoblasts, that is, of polyvalent
cells, capable of many types of histological differentiation? Or rather, are