BLASTOGENESIS AND MORPHOGENESIS
177
only do they converge for the repair of tissues; they can also be the
origin of germ cells. They participate in asexual reproduction and regeneration, where their intervention is very important, sometimes indispensable. However, these cells scattered throughout the organized body
play a role in the life of the animal, in its structure, its histogenesis, its
physiology; they are the elements of repair, of regeneration, the factors
in longevity. They are thus essentially somatic cells. When they intervene
in asexual reproduction and from that time become blastogenic cells,
they regain these reproductive and ontogenic capacities only inside the
organism, and not outside of it. It is in the organism that the physiological
conditions exist which can give these cells their ontogenic ability.
Neoblasts are embryonic cells held in reserve in the adult organism, in
the course of ontogenesis and histogenesis, like the imaginal discs of
insect larvae. They can be eliminated, by irradiation, for example, especially in planarians and hydras. There are cases, however, where the adult
animal can reconstitute them de novo, in hydroids for example (Brien,
1963).
IV. Blastogenesis and Somatic Continuity
Blastogenesis discloses the natural continuity of the soma. Sometimes,
it alone assures survival of the species. It is by their buds, even more than
by their larvae, that certain animals continue to live and that their
species are perpetuated.
On the coasts of the English channel, during winter, colonies of
Clavelina often disappear. But in their place, budding vesicles persist
from which, in the spring, colonies are re-formed. In the lagoons of the
Phlegraean Fields near Naples, during winter, colonies of
Clavelina
phlegrea leave on the substrate only their stolons, surrounded by a thick
layer of tunic, similar to rhizoids. It is from these rhizoids that colonies
rise again in the spring (Brien, 1930).
The Spongillidae and Phylactolaemata die during the winter in temperate regions or in a dry season in the tropics, but their gemmules or
their statoblasts remain, spread out, and develop with the return of
favorable conditions.
Even some animals with intense agamic propagation have lost sexual
reproduction or can do without it. According to Wenzenberg-Lund (1901),
the phylactolaemate, Fredericella sultana, never produces larvae in Denmark. Sexuality is not manifested in the oligochaetes, Aeolosoma
and
Chaetogaster
in experimental culture; they only maintain themselves by
asexual reproduction. For years Korschelt (1942) has raised the poly-
177
only do they converge for the repair of tissues; they can also be the
origin of germ cells. They participate in asexual reproduction and regeneration, where their intervention is very important, sometimes indispensable. However, these cells scattered throughout the organized body
play a role in the life of the animal, in its structure, its histogenesis, its
physiology; they are the elements of repair, of regeneration, the factors
in longevity. They are thus essentially somatic cells. When they intervene
in asexual reproduction and from that time become blastogenic cells,
they regain these reproductive and ontogenic capacities only inside the
organism, and not outside of it. It is in the organism that the physiological
conditions exist which can give these cells their ontogenic ability.
Neoblasts are embryonic cells held in reserve in the adult organism, in
the course of ontogenesis and histogenesis, like the imaginal discs of
insect larvae. They can be eliminated, by irradiation, for example, especially in planarians and hydras. There are cases, however, where the adult
animal can reconstitute them de novo, in hydroids for example (Brien,
1963).
IV. Blastogenesis and Somatic Continuity
Blastogenesis discloses the natural continuity of the soma. Sometimes,
it alone assures survival of the species. It is by their buds, even more than
by their larvae, that certain animals continue to live and that their
species are perpetuated.
On the coasts of the English channel, during winter, colonies of
Clavelina often disappear. But in their place, budding vesicles persist
from which, in the spring, colonies are re-formed. In the lagoons of the
Phlegraean Fields near Naples, during winter, colonies of
Clavelina
phlegrea leave on the substrate only their stolons, surrounded by a thick
layer of tunic, similar to rhizoids. It is from these rhizoids that colonies
rise again in the spring (Brien, 1930).
The Spongillidae and Phylactolaemata die during the winter in temperate regions or in a dry season in the tropics, but their gemmules or
their statoblasts remain, spread out, and develop with the return of
favorable conditions.
Even some animals with intense agamic propagation have lost sexual
reproduction or can do without it. According to Wenzenberg-Lund (1901),
the phylactolaemate, Fredericella sultana, never produces larvae in Denmark. Sexuality is not manifested in the oligochaetes, Aeolosoma
and
Chaetogaster
in experimental culture; they only maintain themselves by
asexual reproduction. For years Korschelt (1942) has raised the poly-