BLASTOGENESIS AND MORPHOGENESIS
163
Precocious scissiparity may be intermittent,
for example in Stylaria
(Fig. 8). The zone of precocious scissiparity does not appear in the reconstituted segment, N, but in the preceding segment, N-l. The b portion
of segment N-l and the a portion of the preceding segment, N, will form
the second blastozooid. The third area of scissiparity appears in segment
N-2, and so on. The body of the parent zooid decreases in size each time.
However, it cannot comprise less than seventeen segments. At that point
all scissiparity stops until a sufficient number of segments is restored.
Precocious scissiparity, therefore, is intermittent in this case.
These examples suffice to demonstrate that asexual reproduction or budding is the result of continuous growth at a specifically localized point,
the blastogenic zone, a zone of such physiological isolation that a return
to ontogenic proliferation occurs.
III. Origin of Blastogenic Cells
At the end of the 19th century, the conception of the duality of the
organism into germ and soma extended to blastogenesis. Braem (1890;
1897, 1908a,b, 1910), to whom we owe very fine monographs on the
phylactolaemates, based on budding of Plumatella,
believed that there
exists in animals a "reserve" of embryonic cells endowed with the same
qualities as those in the germ "line." These would be destined to form
buds. The blastozooid in being constructed does not exhaust them, but
rather, it conserves enough of them to insure its own budding. The
embryonic reserve thus is maintained from blastozooid to blastozooid,
from generation to generation, continuous, immortal, like the germ line.
The theory of the embryonic reserve does not rest on any more fact than
that of the duality of the organism into soma and germ. Braem confused
polypidian organogenesis with true cystidial budding.
The origin of blastogenic cells is actually complex and necessarily
varies according to the type of budding which, itself, depends on the
organization of the budding animal.
The blastozooid zoecia of bryozoans are formed from the wall, as has
been described, always at the same place (Fig. 9). Thus they are derived
from the cells that constitute that wall—ectodermal and pleuroparietal
cells. The ectodermal cells proliferate and form a cellular button from
which all the organs of the polypide are derived: the digestive loop, the
tentacular lophophore, and the nervous system. The pleuroparietal cells
play a more limited role, becoming the coelomic epithelium which lines
all the organs of the polypide and the walls of the new zooecium (Fig.
10).
163
Precocious scissiparity may be intermittent,
for example in Stylaria
(Fig. 8). The zone of precocious scissiparity does not appear in the reconstituted segment, N, but in the preceding segment, N-l. The b portion
of segment N-l and the a portion of the preceding segment, N, will form
the second blastozooid. The third area of scissiparity appears in segment
N-2, and so on. The body of the parent zooid decreases in size each time.
However, it cannot comprise less than seventeen segments. At that point
all scissiparity stops until a sufficient number of segments is restored.
Precocious scissiparity, therefore, is intermittent in this case.
These examples suffice to demonstrate that asexual reproduction or budding is the result of continuous growth at a specifically localized point,
the blastogenic zone, a zone of such physiological isolation that a return
to ontogenic proliferation occurs.
III. Origin of Blastogenic Cells
At the end of the 19th century, the conception of the duality of the
organism into germ and soma extended to blastogenesis. Braem (1890;
1897, 1908a,b, 1910), to whom we owe very fine monographs on the
phylactolaemates, based on budding of Plumatella,
believed that there
exists in animals a "reserve" of embryonic cells endowed with the same
qualities as those in the germ "line." These would be destined to form
buds. The blastozooid in being constructed does not exhaust them, but
rather, it conserves enough of them to insure its own budding. The
embryonic reserve thus is maintained from blastozooid to blastozooid,
from generation to generation, continuous, immortal, like the germ line.
The theory of the embryonic reserve does not rest on any more fact than
that of the duality of the organism into soma and germ. Braem confused
polypidian organogenesis with true cystidial budding.
The origin of blastogenic cells is actually complex and necessarily
varies according to the type of budding which, itself, depends on the
organization of the budding animal.
The blastozooid zoecia of bryozoans are formed from the wall, as has
been described, always at the same place (Fig. 9). Thus they are derived
from the cells that constitute that wall—ectodermal and pleuroparietal
cells. The ectodermal cells proliferate and form a cellular button from
which all the organs of the polypide are derived: the digestive loop, the
tentacular lophophore, and the nervous system. The pleuroparietal cells
play a more limited role, becoming the coelomic epithelium which lines
all the organs of the polypide and the walls of the new zooecium (Fig.
10).