178
PAUL BRIEN
chaete, Ctenodrilus
pardalis,
which has never multiplied except by
schizogony. Vandel (1922) has noted strains of Planaria
subtentaculata
and of Planaria gonocephala which are never sexual, but reproduce by
scissiparity.
V. Blastogenesis and Renewal of Germ Cells
In most of the cases of blastogenesis the blastozooid is identical with
the oozooid, and, just as much as the latter, it can be sexual. The blastozooids of the Clavelinidae, Polyclinidae, Didemnidae, Perophoridae,
Polystyelidae, Botryllidae, those which develop from the budding of
growth of bryozoans, as well as from statoblasts, buds formed by
schizogenesis in annelids and Platyhelminthes, and the young sponge
developing from gemmules, all these blastozooids are capable of reproducing sexually. At a particular time in their growth they will thus be
provided with germ cells. But, these are formed by somatic cells, as was
demonstrated previously.
The planarian in scissiparity can even have its gonad removed, and
then the blastozooids, which are formed from these fragments will be
able to build one de novo (Vandel, 1921). Organogenesis of a young
Clavelina, derived from a bud, takes place from mesenchymal cells of
the parent stolon. It will still have ovaries and testes. Young blastozooids
of Perophora and Stolonica are still destined to reproduce sexually.
Budding confirms, by factual evidence, that there is not an irrefutable
distinction between germ* and soma (Brien, 1926-1964). The evolution
of one to the other is reversible. The germ cells, just like other lines of
somatic cells, are segregated at the time of organogenesis. The germinal
tissue is neither more continuous nor more immortal than any other
tissue. It is recreated in each generation, from the original germinal
tissue, egg or bud, in its own time and place, as a result of the embryonic
physiology and organogenesis.
This conclusion, to which budding leads us in an irrefutable manner,
seems to be in opposition to a principle that is considered fundamental,
namely that castration is not followed by regeneration of gonads.
It is on this principle that operative techniques are based for the
study of sexual physiology. But this is only true in higher animals—
arthropods and vertebrates—exactly in those in which the power of
regeneration is limited and in which the removal of a limb, the nervous
system, or any other organ is no more followed by regeneration than
that of the gonads. Furthermore, experimental restoration of gonads has
been shown in organisms endowed with regulative capacity.
PAUL BRIEN
chaete, Ctenodrilus
pardalis,
which has never multiplied except by
schizogony. Vandel (1922) has noted strains of Planaria
subtentaculata
and of Planaria gonocephala which are never sexual, but reproduce by
scissiparity.
V. Blastogenesis and Renewal of Germ Cells
In most of the cases of blastogenesis the blastozooid is identical with
the oozooid, and, just as much as the latter, it can be sexual. The blastozooids of the Clavelinidae, Polyclinidae, Didemnidae, Perophoridae,
Polystyelidae, Botryllidae, those which develop from the budding of
growth of bryozoans, as well as from statoblasts, buds formed by
schizogenesis in annelids and Platyhelminthes, and the young sponge
developing from gemmules, all these blastozooids are capable of reproducing sexually. At a particular time in their growth they will thus be
provided with germ cells. But, these are formed by somatic cells, as was
demonstrated previously.
The planarian in scissiparity can even have its gonad removed, and
then the blastozooids, which are formed from these fragments will be
able to build one de novo (Vandel, 1921). Organogenesis of a young
Clavelina, derived from a bud, takes place from mesenchymal cells of
the parent stolon. It will still have ovaries and testes. Young blastozooids
of Perophora and Stolonica are still destined to reproduce sexually.
Budding confirms, by factual evidence, that there is not an irrefutable
distinction between germ* and soma (Brien, 1926-1964). The evolution
of one to the other is reversible. The germ cells, just like other lines of
somatic cells, are segregated at the time of organogenesis. The germinal
tissue is neither more continuous nor more immortal than any other
tissue. It is recreated in each generation, from the original germinal
tissue, egg or bud, in its own time and place, as a result of the embryonic
physiology and organogenesis.
This conclusion, to which budding leads us in an irrefutable manner,
seems to be in opposition to a principle that is considered fundamental,
namely that castration is not followed by regeneration of gonads.
It is on this principle that operative techniques are based for the
study of sexual physiology. But this is only true in higher animals—
arthropods and vertebrates—exactly in those in which the power of
regeneration is limited and in which the removal of a limb, the nervous
system, or any other organ is no more followed by regeneration than
that of the gonads. Furthermore, experimental restoration of gonads has
been shown in organisms endowed with regulative capacity.