BLASTOGENESIS AND MORPHOGENESIS
181
Is it necessary to recall the information that hydras provide on this
subject? The germ cells come from interstitial cells. In Hydra fusca and
Hydra pirardi, at a temperature of 19°C, the interstitial cells are exclusively somatic histoblasts; they assure the continuous growth of the
polyp which is propagated solely by budding. On the other hand, at 8°C,
they abruptly change their direction, and are diverted from their original
function; they become germ cells. From that time on the hydra slows
or ceases its growth and budding, is weakened, and dies. When a hydra in
gametogenesis at 8°C is brought back to 19°C, the spermatocytes and
ovocytes undergo histolysis and the gonia return to the condition of
interstitial cells. The latter resume their somatic role, the hydra recommences its growth, and continuous budding. The interstitial cells are,
thus, depending on the physiological condition of the polyp, which is
itself conditioned by the environment, in sequence and in a reversible
fashion, somatic cells or germ cells (Brien, 1949, 1964, 1966).
Often blastogenic cells and germ cells have the same origin. In the
Spongillidae, the amebocytes will be the archaeocytes of the gemmules
or the mother cells of the gonia. In the manubrial budding of certain
medusae, especially Limnocnida
tanganikae,
the germ cells are formed
in the manubrium, in the region which is also the budding area, and from
basal embryonic cells of the ectoderm which are also blastogenic cells
(Bouillon, 1957). The resemblance is still more complete in the medusae
of Margelidae, where the manubrial budding of the medusa comes
exclusively from basal embryonic cells of the ectoderm which, in the
FIG. 15. Budding of Metandrocarpa
taylori (polystyelid tunicate). Fragment of
one of the extensions of the tunic (Tu.) radiating out from the base of a budding
ascidiozooid which is, itself, bifurcating. The extension of the tunic envelops the
vessels of the test which are seen on the basal surface and which enclose between
them the blastozooids (Asz.), formed one from another, and as a result arranged
in order of age along the extension of the tunic. The youngest and most marginal
blastozooids put forth blastogenic swellings (B.) which will become new blastozooid-ascidiozooids. This budding is conditioned by the vessels of the test which
form an anastomozing network, are open at the base of each ascidiozooid. The
network is prolonged by a crown of pulsating ampullae. These contribute to the
elongation of the marginal tunic during their turgescence, and by their extension,
participate in peduncle formation, then in the spacing of the new blastozooid. Each
bud is formed by a dilatation of the stolon. Consequently, it is composed of the
same elements: the ectoderm
being derived from the ectoderm of the parent
ascidiozooid, an internal vesicle extending into the stolon from the parent peribranchial wall; blood mesenchyme is derived from the parent. (After Andrew
Todd Newberry.)
181
Is it necessary to recall the information that hydras provide on this
subject? The germ cells come from interstitial cells. In Hydra fusca and
Hydra pirardi, at a temperature of 19°C, the interstitial cells are exclusively somatic histoblasts; they assure the continuous growth of the
polyp which is propagated solely by budding. On the other hand, at 8°C,
they abruptly change their direction, and are diverted from their original
function; they become germ cells. From that time on the hydra slows
or ceases its growth and budding, is weakened, and dies. When a hydra in
gametogenesis at 8°C is brought back to 19°C, the spermatocytes and
ovocytes undergo histolysis and the gonia return to the condition of
interstitial cells. The latter resume their somatic role, the hydra recommences its growth, and continuous budding. The interstitial cells are,
thus, depending on the physiological condition of the polyp, which is
itself conditioned by the environment, in sequence and in a reversible
fashion, somatic cells or germ cells (Brien, 1949, 1964, 1966).
Often blastogenic cells and germ cells have the same origin. In the
Spongillidae, the amebocytes will be the archaeocytes of the gemmules
or the mother cells of the gonia. In the manubrial budding of certain
medusae, especially Limnocnida
tanganikae,
the germ cells are formed
in the manubrium, in the region which is also the budding area, and from
basal embryonic cells of the ectoderm which are also blastogenic cells
(Bouillon, 1957). The resemblance is still more complete in the medusae
of Margelidae, where the manubrial budding of the medusa comes
exclusively from basal embryonic cells of the ectoderm which, in the
FIG. 15. Budding of Metandrocarpa
taylori (polystyelid tunicate). Fragment of
one of the extensions of the tunic (Tu.) radiating out from the base of a budding
ascidiozooid which is, itself, bifurcating. The extension of the tunic envelops the
vessels of the test which are seen on the basal surface and which enclose between
them the blastozooids (Asz.), formed one from another, and as a result arranged
in order of age along the extension of the tunic. The youngest and most marginal
blastozooids put forth blastogenic swellings (B.) which will become new blastozooid-ascidiozooids. This budding is conditioned by the vessels of the test which
form an anastomozing network, are open at the base of each ascidiozooid. The
network is prolonged by a crown of pulsating ampullae. These contribute to the
elongation of the marginal tunic during their turgescence, and by their extension,
participate in peduncle formation, then in the spacing of the new blastozooid. Each
bud is formed by a dilatation of the stolon. Consequently, it is composed of the
same elements: the ectoderm
being derived from the ectoderm of the parent
ascidiozooid, an internal vesicle extending into the stolon from the parent peribranchial wall; blood mesenchyme is derived from the parent. (After Andrew
Todd Newberry.)