BLASTOGENESIS AND MORPHOGENESIS
165
Whatever variations the budding of tunicates may show, the bud is
always a triploblastic vesicle of the gastrula type with three basic layers:
ectoblast, endoblast, and mesoblast between the other two (Fig. 11). In
other cases of budding, other elements are added, not only trophic materials, which are used up as the blastozooid is formed, but also various
blastogenic constituents which take part, along with the three basic
layers, in the ontogenesis of the new animal. These new constituents are
always fragments of organs from the parent. Their interference in organogenesis results in limiting the role of the basic layers in the exact amount
by which they take over for them, according to one of the best examples
of the correlations, inductions, and inhibitions which assure the morphological unity of the new organism (discussed below).
In this triploblastic bud the ectoblast gives rise exclusively to the
ectoderm of the blastozooid. By contrast, the endoblast has multiple
potentialities. It forms most of the organs: the pharynx, the peripharyngeal cavities, the digestive loop, the epicardium, the heart, and often the
neuroglandular system. The mesoblast is destined to differentiate into the
mesenchyme of the hemocoel, to produce the cellular elements of the
blood and to form the musculature. Now, what is the origin of these three
layers of the bud? It is different according to the group of tunicates
under consideration. In Clavelina the ectoblast originates from the ectoderm of the stolon, the endoblast as well as the mesoblast, from the
mesenchyme of the stolon. And so, in Clavelina, it is the mesenchmyal
cells from the stolon which reacquire the most extensive organogenic
possibilities.
In the bud of the Polyclinidae, the ectoblast is derived from postabdominal ectoderm; the endoblast is represented by a fragment of the
epicardium, which, being itself a prolongation of the pharynx, is endodermal. The mesoblast is provided by the mesenchyme of the postabdomen. In the postabdominal budding of the Polyclinidae, the fragment of
the epicardium, representing the endoblast of the bud, regains the most
extensive organogenic possibilities.
FIG. 9. Zooecium of Paludicella
lacrustis (Gymnolaemata) from which the
polypide extends out its tentacular lophophore from the peristome of the cystid.
The cystid is putting forth a dorsal swelling which is the cystidian bud. This young
cystid is closed by a septum, and its ectomesodermal wall forms the polypidian bud
from which its polypide is constructed. Cy.—cystid; Po.—polypide; Per.—peristome;
Bo.—mouth; An.—anus; L. T.—tentacular lophophore; B. Cy.—cystidian bud; B.
Po.—polypidian bud. (Brien, 1960.)
165
Whatever variations the budding of tunicates may show, the bud is
always a triploblastic vesicle of the gastrula type with three basic layers:
ectoblast, endoblast, and mesoblast between the other two (Fig. 11). In
other cases of budding, other elements are added, not only trophic materials, which are used up as the blastozooid is formed, but also various
blastogenic constituents which take part, along with the three basic
layers, in the ontogenesis of the new animal. These new constituents are
always fragments of organs from the parent. Their interference in organogenesis results in limiting the role of the basic layers in the exact amount
by which they take over for them, according to one of the best examples
of the correlations, inductions, and inhibitions which assure the morphological unity of the new organism (discussed below).
In this triploblastic bud the ectoblast gives rise exclusively to the
ectoderm of the blastozooid. By contrast, the endoblast has multiple
potentialities. It forms most of the organs: the pharynx, the peripharyngeal cavities, the digestive loop, the epicardium, the heart, and often the
neuroglandular system. The mesoblast is destined to differentiate into the
mesenchyme of the hemocoel, to produce the cellular elements of the
blood and to form the musculature. Now, what is the origin of these three
layers of the bud? It is different according to the group of tunicates
under consideration. In Clavelina the ectoblast originates from the ectoderm of the stolon, the endoblast as well as the mesoblast, from the
mesenchyme of the stolon. And so, in Clavelina, it is the mesenchmyal
cells from the stolon which reacquire the most extensive organogenic
possibilities.
In the bud of the Polyclinidae, the ectoblast is derived from postabdominal ectoderm; the endoblast is represented by a fragment of the
epicardium, which, being itself a prolongation of the pharynx, is endodermal. The mesoblast is provided by the mesenchyme of the postabdomen. In the postabdominal budding of the Polyclinidae, the fragment of
the epicardium, representing the endoblast of the bud, regains the most
extensive organogenic possibilities.
FIG. 9. Zooecium of Paludicella
lacrustis (Gymnolaemata) from which the
polypide extends out its tentacular lophophore from the peristome of the cystid.
The cystid is putting forth a dorsal swelling which is the cystidian bud. This young
cystid is closed by a septum, and its ectomesodermal wall forms the polypidian bud
from which its polypide is constructed. Cy.—cystid; Po.—polypide; Per.—peristome;
Bo.—mouth; An.—anus; L. T.—tentacular lophophore; B. Cy.—cystidian bud; B.
Po.—polypidian bud. (Brien, 1960.)