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PAUL BRIEN
thorax. Whichever it may be, organogenesis is completed by morphallaxis,
by an adjustment or readjustment of young or old thoracic parts to the
corresponding abdominal parts.
The epicardium, therefore, is an organ of very variable organogenic
power, extensive or restricted according to the importance of the inhibition exercised by the primordia that participate in the construction of
the blastozooid. Although they may be extensive or restricted, these
potentialities are always complementary to other blastogenic elements
in such a way that the organs of the ascidiozooid respond with the
adjustments which are necessary and sufficient for it to survive and grow.
Just as inhibitions occur between the blastomeres of a segmenting egg,
between the morphogenic territories of a gastrula, between the primordia
of the embryo, so too the elements forming a bud inhibit each other.
These inhibitions are at the same time inductions of such a nature
that they are limited in their potential for proliferation and differentiation in such a proportion that their histogenesis and organogenesis may
join in the completion of the morphological and physiological unity of
a young functional animal.
VIII. Double Ontogenesis or Multiple Ontogenesis
The blastozooid developed from a bud is representative of the species
as much as the oozooid. Most often, in one species, blastozooid and oozooid are identical. In a colony of Clavelina or of Polyclinidae, they cannot
be distinguished.
However, ontogenesis of a blastozooid is accomplished by very different
organogenic processes from those of embryogenesis.
The bryozoan egg gives rise to a free-swimming planktonic larva, the
cyphonautes, with a structure similar to the trochophore of polychaetes,
but often simpler. However that may be, its embryogenesis is that of a
polychaete trochophore, somewhat degenerated. After a certain time of
free life, the larva attaches to a submerged substrate, and from then on
undergoes a curious metamorphosis. The trochophore-like structure disintegrates; the larva subsists only in the form of a small closed sac fixed
to its substrate. It is the primordial cystid, the wall of which is twolayered ectomesoblast. Although the larval organs disappear at a very
precise point the wall of the cystid produces the polypid bud which
re-forms the organs missing and indispensable to the life of the cystid:
the digestive loop, the tentacular lophophore, the nervous system, etc.,
of the zooecial founder of the colony. Without going into more details,
which can be found in all textbooks of zoology, what has just been briefly
PAUL BRIEN
thorax. Whichever it may be, organogenesis is completed by morphallaxis,
by an adjustment or readjustment of young or old thoracic parts to the
corresponding abdominal parts.
The epicardium, therefore, is an organ of very variable organogenic
power, extensive or restricted according to the importance of the inhibition exercised by the primordia that participate in the construction of
the blastozooid. Although they may be extensive or restricted, these
potentialities are always complementary to other blastogenic elements
in such a way that the organs of the ascidiozooid respond with the
adjustments which are necessary and sufficient for it to survive and grow.
Just as inhibitions occur between the blastomeres of a segmenting egg,
between the morphogenic territories of a gastrula, between the primordia
of the embryo, so too the elements forming a bud inhibit each other.
These inhibitions are at the same time inductions of such a nature
that they are limited in their potential for proliferation and differentiation in such a proportion that their histogenesis and organogenesis may
join in the completion of the morphological and physiological unity of
a young functional animal.
VIII. Double Ontogenesis or Multiple Ontogenesis
The blastozooid developed from a bud is representative of the species
as much as the oozooid. Most often, in one species, blastozooid and oozooid are identical. In a colony of Clavelina or of Polyclinidae, they cannot
be distinguished.
However, ontogenesis of a blastozooid is accomplished by very different
organogenic processes from those of embryogenesis.
The bryozoan egg gives rise to a free-swimming planktonic larva, the
cyphonautes, with a structure similar to the trochophore of polychaetes,
but often simpler. However that may be, its embryogenesis is that of a
polychaete trochophore, somewhat degenerated. After a certain time of
free life, the larva attaches to a submerged substrate, and from then on
undergoes a curious metamorphosis. The trochophore-like structure disintegrates; the larva subsists only in the form of a small closed sac fixed
to its substrate. It is the primordial cystid, the wall of which is twolayered ectomesoblast. Although the larval organs disappear at a very
precise point the wall of the cystid produces the polypid bud which
re-forms the organs missing and indispensable to the life of the cystid:
the digestive loop, the tentacular lophophore, the nervous system, etc.,
of the zooecial founder of the colony. Without going into more details,
which can be found in all textbooks of zoology, what has just been briefly