BLASTOGENESIS AND MORPHOGENESIS
157
and always on the side corresponding to the ventral surface, daughter
zooecia grow out successively from place to place, the oldest being in the
lowest position, the younger ones more and more anterior. Each daughter
zooecium buds according to the same law, along the branch formed by
the growth of the parent zooecium. Each of them becomes, actually, a
cluster of zooecia of various generations. These clusters of zooecia are
spaced out along the branch formed by the parent zooecium (Brien,
1960).
Among the tunicates the same findings are noted. In Clavelina
lepadiformis, budding is from the stolon. At the same time and at the same
rate as the stolen elongates, branching out on its substrate, the circulation becomes more and more difficult, then ceases, inducing an embolus
of leukocytes in a lobulated varicose swelling. Finally this amputates
itself; it develops into a budding vesicle where the original mesenchyme,
nourished by the accumulated leukocytes undergoing histolysis, becomes
blastogenic, and forms the organs of a blastozooid. The amputated stolon
resumes its growth until the same phenomenon is repeated. The result is
that around the parent Clavelina, the substrate to which it adheres is
covered with budding vesicles (Brien, 1928).
The Polyclinidae furnish an especially clear demonstration of the same
principle. They are essentially colonial. The ascidiozooids are arranged in
small groups or systems or "cenobies" where they all have the same
orientation. These very regular cenobies are themselves covered with a
common tunic which constitutes the morphological and physiological link
between all the ascidiozooids. The entire colony adheres to the substrate.
It is brightly colored, resembling a strawberry. Sometimes it has a stemlike base, with the appearance of a mushroom. The postabdomen of each
ascidiozooid grows longer and longer, thus stretching out the heart and
pericardial cavity, making circulation more and more difficult. Soon the
swollen postabdomen cuts itself off from the abdomen (Fig. 4). The
ascidiozooid, thus cut off from its postabdomen, is immediately reconstituted at the expense of the fragment of epicardium which remains, and
reforms the heart and pericardial cavity. This reestablishes the circulation of blood, allowing growth and elongation to take place, until a new
amputation occurs. The postabdomen separated from the abdomen of the
ascidiozooid remains in a common colonial tunic, then undergoes strobilation. The strobilation takes place progressively from inferior to superior,
the first strobila separated being the most posterior. Each of these
strobilae is a bud (Brien, 1948).
The ascidiozooid of Perophora listeri also sends out a stolon which
157
and always on the side corresponding to the ventral surface, daughter
zooecia grow out successively from place to place, the oldest being in the
lowest position, the younger ones more and more anterior. Each daughter
zooecium buds according to the same law, along the branch formed by
the growth of the parent zooecium. Each of them becomes, actually, a
cluster of zooecia of various generations. These clusters of zooecia are
spaced out along the branch formed by the parent zooecium (Brien,
1960).
Among the tunicates the same findings are noted. In Clavelina
lepadiformis, budding is from the stolon. At the same time and at the same
rate as the stolen elongates, branching out on its substrate, the circulation becomes more and more difficult, then ceases, inducing an embolus
of leukocytes in a lobulated varicose swelling. Finally this amputates
itself; it develops into a budding vesicle where the original mesenchyme,
nourished by the accumulated leukocytes undergoing histolysis, becomes
blastogenic, and forms the organs of a blastozooid. The amputated stolon
resumes its growth until the same phenomenon is repeated. The result is
that around the parent Clavelina, the substrate to which it adheres is
covered with budding vesicles (Brien, 1928).
The Polyclinidae furnish an especially clear demonstration of the same
principle. They are essentially colonial. The ascidiozooids are arranged in
small groups or systems or "cenobies" where they all have the same
orientation. These very regular cenobies are themselves covered with a
common tunic which constitutes the morphological and physiological link
between all the ascidiozooids. The entire colony adheres to the substrate.
It is brightly colored, resembling a strawberry. Sometimes it has a stemlike base, with the appearance of a mushroom. The postabdomen of each
ascidiozooid grows longer and longer, thus stretching out the heart and
pericardial cavity, making circulation more and more difficult. Soon the
swollen postabdomen cuts itself off from the abdomen (Fig. 4). The
ascidiozooid, thus cut off from its postabdomen, is immediately reconstituted at the expense of the fragment of epicardium which remains, and
reforms the heart and pericardial cavity. This reestablishes the circulation of blood, allowing growth and elongation to take place, until a new
amputation occurs. The postabdomen separated from the abdomen of the
ascidiozooid remains in a common colonial tunic, then undergoes strobilation. The strobilation takes place progressively from inferior to superior,
the first strobila separated being the most posterior. Each of these
strobilae is a bud (Brien, 1948).
The ascidiozooid of Perophora listeri also sends out a stolon which