BLASTOGENESIS AND MORPHOGENESIS
187
Thus, at a single level, the epicardium can form everything, as in an
isolated budding strobila. But its multiple potentialities are more or less
restricted according to the polarity imposed upon it by the organs present
in the sectioned part. It is this polarity which gives the organogenic
orientation to the regenerating elements.
The same observation can be made concerning a section in the esophageal region of Clavelina (Fig. 19). At a single level, the epicardial surface
on the anterior part which has a thorax will form exclusively cardiopericardial organ. The epicardial surface on the side containing the
abdomen will produce all the organs of the thorax. What will be the
polarity of an esophageal fragment entirely isolated by two simultaneous
cuts, one anterior, the other posterior? The epicardium has the same
morphogenic orientation at its two cut surfaces. Two complete thoraxes
are formed at the two extremities of the esophageal part. Regeneration
is bipolar. It is heteromorphic.
On the contrary, if the anterior cut is made 72 hours before the
posterior cut, the epicardium has already formed a thoracic primodium
sufficiently advanced to impose a polarity so that the epicardium, at the
time of the posterior cut, is inhibited at that level and can form only the
cardiopericardial organ. It may be that the polarity of the strobila of
Polyclinidae can be explained, at least partially, in the same manner.
Perhaps the gradient in natural autotomy permits the commencement
of organogenesis at the anterior extremity of each epicardial fragment,
to such a degree that the posterior extremity is limited to forming the
heart (Brien, 1932).
VII. Correlative Competition—Total Potentiality and
Restricted Potentiality
From the preceding, it follows that the constituent elements of a bud
are more or less complex and heterogeneous. In its simplest organization
FIG. 19. Cross sections in the esophageal region of Clavelina lepadiformis.
I.
Abdominal region of Clavelina:
digestive loop, epicardium and cardiopericardial
organ are shown, as well as the level of two sections: anterior a-b; posterior c-d.
II. After the first cut, separating abdomen from thorax, a new thorax regenerates, its
organs formed exclusively by the epicardium. III. Fragment of isolated esophagus
containing an epicardial part (Ep.), esophagus (Oe.), intestine (R.). (A) The two
cross sections are synchronous; bipolar regeneration forms a thorax at both
extremities. (B) The anterior cut, a-b, precedes the posterior one, c-d, by 72 hours;
regeneration is unipolar; the thorax is outlined anteriorly, impressing a polarity on
the esophageal fragment such that at the posterior the epicardium forms the cardiopericardial organ (Brien, 1932).
187
Thus, at a single level, the epicardium can form everything, as in an
isolated budding strobila. But its multiple potentialities are more or less
restricted according to the polarity imposed upon it by the organs present
in the sectioned part. It is this polarity which gives the organogenic
orientation to the regenerating elements.
The same observation can be made concerning a section in the esophageal region of Clavelina (Fig. 19). At a single level, the epicardial surface
on the anterior part which has a thorax will form exclusively cardiopericardial organ. The epicardial surface on the side containing the
abdomen will produce all the organs of the thorax. What will be the
polarity of an esophageal fragment entirely isolated by two simultaneous
cuts, one anterior, the other posterior? The epicardium has the same
morphogenic orientation at its two cut surfaces. Two complete thoraxes
are formed at the two extremities of the esophageal part. Regeneration
is bipolar. It is heteromorphic.
On the contrary, if the anterior cut is made 72 hours before the
posterior cut, the epicardium has already formed a thoracic primodium
sufficiently advanced to impose a polarity so that the epicardium, at the
time of the posterior cut, is inhibited at that level and can form only the
cardiopericardial organ. It may be that the polarity of the strobila of
Polyclinidae can be explained, at least partially, in the same manner.
Perhaps the gradient in natural autotomy permits the commencement
of organogenesis at the anterior extremity of each epicardial fragment,
to such a degree that the posterior extremity is limited to forming the
heart (Brien, 1932).
VII. Correlative Competition—Total Potentiality and
Restricted Potentiality
From the preceding, it follows that the constituent elements of a bud
are more or less complex and heterogeneous. In its simplest organization
FIG. 19. Cross sections in the esophageal region of Clavelina lepadiformis.
I.
Abdominal region of Clavelina:
digestive loop, epicardium and cardiopericardial
organ are shown, as well as the level of two sections: anterior a-b; posterior c-d.
II. After the first cut, separating abdomen from thorax, a new thorax regenerates, its
organs formed exclusively by the epicardium. III. Fragment of isolated esophagus
containing an epicardial part (Ep.), esophagus (Oe.), intestine (R.). (A) The two
cross sections are synchronous; bipolar regeneration forms a thorax at both
extremities. (B) The anterior cut, a-b, precedes the posterior one, c-d, by 72 hours;
regeneration is unipolar; the thorax is outlined anteriorly, impressing a polarity on
the esophageal fragment such that at the posterior the epicardium forms the cardiopericardial organ (Brien, 1932).