BLASTOGENESIS AND MORPHOGENESIS
179
The observation of Vandel (1921) has already been mentioned that
planarians originating from fragments of planarians containing no trace
of germinal elements are sexual.
In the polyps of Clava squamata in a sexual stage, the germ cells are
concentrated in the gonophores arranged under the hypostome in thick
whorls. If this region of the gonophores is cut away, the reserve germinal
tissue of the polyp is removed at the same time. However, after cicatrization, the two layers of the wall reconstruct as well as hypostome and its
tentacles. New gonophores appear. The reconstituted ectodermal epithelium forms interstitial cells. At the base of the hypostome these cells
invade the endoderm, become gonia, and migrate into the new gonophores.
All the germinal and sexual structures of the polyps are thus formed anew
from the somatic elements of the polyp (Brien, 1943).
A similar renewal of germinal tissue has been induced in
Lumbricillus
lineatus by Herlant-Meewis (1946). A cut is made at the level of the
eighth segment in front of the genital segments; the regeneration of the
anterior trunk, so short, is difficult, but possible. In the regenerates
obtained, the segments posterior to the eighth are thus reconstituted, and
among them, the genital segments. Herlant-Meewis has been able to
follow the reappearance there of gonia from somatic cells of the coelomic
layer.
LaViolette (1954) has obtained the reconstitution of the genital
gland after experimental removal in Avion, It redifferentiates from fragments of genital organ, somatic cells remaining in place in the course
of the ablation.
The Polystyelidae, a family of tunicates, shows parietal budding, e.g.,
Stolonica, Metandrocarpa,
Polycarpa, Distomus
(Fig. 15). From the base
of the ascidiozooid, stolons spread out which are formed by two long
tubular swellings, joined together, derived from the ectodermal and
peribranchial layers, between which are interposed blood cells and
mesenchyme from the parent. The stolon is surrounded by a sleeve of the
tunic in which the blood vessels of the test ramify. The bud is a dilatation
of the stolon which undergoes autotomy. When organogenesis of the bud
is sufficiently advanced, the germ cells differentiate within it de novo
from blood lymphocytes. Their appearance occurs at the time of organogenesis of the gonad which has been studied recently by Newberry (1965).
This author has been able to show that germ cells, ovogonia, ovocytes,
spermatogonia, spermatocytes, are not more permanent than the gonads
themselves. The ovaries and testes follow the sexual cycle; they appear
and disappear with it (Newberry, 1965).
179
The observation of Vandel (1921) has already been mentioned that
planarians originating from fragments of planarians containing no trace
of germinal elements are sexual.
In the polyps of Clava squamata in a sexual stage, the germ cells are
concentrated in the gonophores arranged under the hypostome in thick
whorls. If this region of the gonophores is cut away, the reserve germinal
tissue of the polyp is removed at the same time. However, after cicatrization, the two layers of the wall reconstruct as well as hypostome and its
tentacles. New gonophores appear. The reconstituted ectodermal epithelium forms interstitial cells. At the base of the hypostome these cells
invade the endoderm, become gonia, and migrate into the new gonophores.
All the germinal and sexual structures of the polyps are thus formed anew
from the somatic elements of the polyp (Brien, 1943).
A similar renewal of germinal tissue has been induced in
Lumbricillus
lineatus by Herlant-Meewis (1946). A cut is made at the level of the
eighth segment in front of the genital segments; the regeneration of the
anterior trunk, so short, is difficult, but possible. In the regenerates
obtained, the segments posterior to the eighth are thus reconstituted, and
among them, the genital segments. Herlant-Meewis has been able to
follow the reappearance there of gonia from somatic cells of the coelomic
layer.
LaViolette (1954) has obtained the reconstitution of the genital
gland after experimental removal in Avion, It redifferentiates from fragments of genital organ, somatic cells remaining in place in the course
of the ablation.
The Polystyelidae, a family of tunicates, shows parietal budding, e.g.,
Stolonica, Metandrocarpa,
Polycarpa, Distomus
(Fig. 15). From the base
of the ascidiozooid, stolons spread out which are formed by two long
tubular swellings, joined together, derived from the ectodermal and
peribranchial layers, between which are interposed blood cells and
mesenchyme from the parent. The stolon is surrounded by a sleeve of the
tunic in which the blood vessels of the test ramify. The bud is a dilatation
of the stolon which undergoes autotomy. When organogenesis of the bud
is sufficiently advanced, the germ cells differentiate within it de novo
from blood lymphocytes. Their appearance occurs at the time of organogenesis of the gonad which has been studied recently by Newberry (1965).
This author has been able to show that germ cells, ovogonia, ovocytes,
spermatogonia, spermatocytes, are not more permanent than the gonads
themselves. The ovaries and testes follow the sexual cycle; they appear
and disappear with it (Newberry, 1965).