190
PAUL BRIEN
ectoblast of the young blastozooid, while the cells of the mesoblastic
mass, all identical to each other, will differentiate to form all the tissues
and organs of the blastozooid.
The bud of a fragment of postabdomen of Polyclinidae is limited by
a part of the postabdominal ectoderm, and contains a segment of the
ovotestis of the parent ascidiozooid which will provide the testes and
ovaries, by a little mesenchyme and a piece of epicardium. In this case
the mesenchyme, so powerful in the bud of Clavelina, has only a very
limited role, that of forming the mesenchyme of the hemocoel of the
blastozooid. The epicardial stem takes its place in the construction of
organs. It is the structure with the greatest potentiality. It has been
noted already that in Aplidium zostericola, strobilation affects not only
the epicardium but also the digestive loop, of which the stomach region is
greatly elongated. The bud thus contains, besides fragments of ovotestis,
mesenchyme, and epicardium, a portion of stomach and intestine. These
two parts of the digestive tube will reconstruct the digestive loop of the
young blastozooid. The potentialities of the epicardium are reduced
proportionately.
The budding of Didemnidae offers an example, no less striking, of
these correlative organogenic inductions and inhibitions. In this highly
specialized group of Aplousorbranches,
the abdomen is small, globular,
and is connected with the thorax, which is reduced to three or four rows
of stigmata, by a narrow peduncle. The epicardia are separated from the
pharynx and remain independent from each other. They extend into the
abdomen in two closed tubes, to the right and left of the mid-ventral line
under the endostyle (Fig. 22). The peduncular zone is the seat of blastogenesis. This budding process is double.
Two buds, while forming, grow in opposite directions, with the axis of
growth crossing the anterior-posterior axis of the parent ascidiozooid.
One of the buds appears under the peduncle on the ventral surface; it is
the thoracic bud. The other is under the peduncle but on the dorsal side;
it is the abdominal bud.
The thoracic bud forms the ascidiozooid thorax with all the organs
pertaining to it: branchial pharynx, peripharyngeal cavities, nervous
system, esophageal siphon. It is derived exclusively from two dilatations,
each one from two epicardia, in the manner of two ventral swellings.
The abdominal bud forms the ascidiozooid abdomen. It contains the
digestive loop, derived from the esophageal wall of the parent. But in
order for the abdomen to be complete it must also have a cardioperi-
PAUL BRIEN
ectoblast of the young blastozooid, while the cells of the mesoblastic
mass, all identical to each other, will differentiate to form all the tissues
and organs of the blastozooid.
The bud of a fragment of postabdomen of Polyclinidae is limited by
a part of the postabdominal ectoderm, and contains a segment of the
ovotestis of the parent ascidiozooid which will provide the testes and
ovaries, by a little mesenchyme and a piece of epicardium. In this case
the mesenchyme, so powerful in the bud of Clavelina, has only a very
limited role, that of forming the mesenchyme of the hemocoel of the
blastozooid. The epicardial stem takes its place in the construction of
organs. It is the structure with the greatest potentiality. It has been
noted already that in Aplidium zostericola, strobilation affects not only
the epicardium but also the digestive loop, of which the stomach region is
greatly elongated. The bud thus contains, besides fragments of ovotestis,
mesenchyme, and epicardium, a portion of stomach and intestine. These
two parts of the digestive tube will reconstruct the digestive loop of the
young blastozooid. The potentialities of the epicardium are reduced
proportionately.
The budding of Didemnidae offers an example, no less striking, of
these correlative organogenic inductions and inhibitions. In this highly
specialized group of Aplousorbranches,
the abdomen is small, globular,
and is connected with the thorax, which is reduced to three or four rows
of stigmata, by a narrow peduncle. The epicardia are separated from the
pharynx and remain independent from each other. They extend into the
abdomen in two closed tubes, to the right and left of the mid-ventral line
under the endostyle (Fig. 22). The peduncular zone is the seat of blastogenesis. This budding process is double.
Two buds, while forming, grow in opposite directions, with the axis of
growth crossing the anterior-posterior axis of the parent ascidiozooid.
One of the buds appears under the peduncle on the ventral surface; it is
the thoracic bud. The other is under the peduncle but on the dorsal side;
it is the abdominal bud.
The thoracic bud forms the ascidiozooid thorax with all the organs
pertaining to it: branchial pharynx, peripharyngeal cavities, nervous
system, esophageal siphon. It is derived exclusively from two dilatations,
each one from two epicardia, in the manner of two ventral swellings.
The abdominal bud forms the ascidiozooid abdomen. It contains the
digestive loop, derived from the esophageal wall of the parent. But in
order for the abdomen to be complete it must also have a cardioperi-