170
PAUL BRIEN
cells become strongly basophilic, enriched with ribonucleic acid; the
nucleus is swollen,, paler, and has a large nucleolus. They regain great
ability for proliferation (Brien, 1941).
However, it must be emphasized that the dedifferentiation preparatory
to blastogenesis reaches somatic cells which, being initially engaged in
somatic structure and function, have not undergone a very marked
histological specialization. The highly differentiated cells, such as muscle
cells and leukocytes, often undergo histolysis in the bud. They provide the
nutritive supply necessary for organogenesis, that which de Selys Longchamps (1917) calls pseudovitellus,
with which the scattered buds are
often abundantly supplied, e.g., leukocytes accumulated in the budding
vesicles of Clavelina and leukocytes and testicular follicles in the buds
of the Polyclinidae.
Sometimes it is the blastogenic cells themselves which demonstrate
vitellogenesis, such as the ovocyte. Such is also the case for the archaeocytes of sponge gemmules and the mesoblastic cells of phylactolaemate
statoblasts.
Furthermore, in all animal organization there are some cells which
retain to some degree an embryonic character, even while having a
specific orientation and histological fate—these are the replacement cells
to which names have been given such as histoblast and blastocyte. In
these, the functioning organism possesses the elements necessary to
compensate for death of cells and to repair tissues, worn by vital activities. Thus, these histoblasts are evidently designed to play a role in the
regeneration of the blastozooid. The histoblasts or blastocytes actually
are related to various tissues and have their particular histological
orientation. Herlant-Meewis (1964) notes their presence in ectoderm and
endoderm as well as mesoderm in Aeolosoma. The blastocysts of these
three layers become very active in the zone of scissiparity. Soon the
differentiated cells of the three layers become dedifferentiated and are
active in their turn. All these cells together form the three cell lines of the
blastema concerned with renewal of organogenesis. At this same level of
scissiparity, for structures related to the bud, the ectoblast derived from
ectoderm forms the head of the cerebral ganglia, the circumpharyngeal
connectives, the subpharyngeal ganglion, the bristle-bearing follicles of
the pharyngeal region, and the stomodaeum which unites with the neopharynx formed by the endoblast. The mesoblast forms the septa,
muscles, and nephridia of the anterior part of the blastozooid. In this
area of scissiparity, there is produced a remodeling of all the tissues
which resume their respective functions in the formation of a new zooid.
PAUL BRIEN
cells become strongly basophilic, enriched with ribonucleic acid; the
nucleus is swollen,, paler, and has a large nucleolus. They regain great
ability for proliferation (Brien, 1941).
However, it must be emphasized that the dedifferentiation preparatory
to blastogenesis reaches somatic cells which, being initially engaged in
somatic structure and function, have not undergone a very marked
histological specialization. The highly differentiated cells, such as muscle
cells and leukocytes, often undergo histolysis in the bud. They provide the
nutritive supply necessary for organogenesis, that which de Selys Longchamps (1917) calls pseudovitellus,
with which the scattered buds are
often abundantly supplied, e.g., leukocytes accumulated in the budding
vesicles of Clavelina and leukocytes and testicular follicles in the buds
of the Polyclinidae.
Sometimes it is the blastogenic cells themselves which demonstrate
vitellogenesis, such as the ovocyte. Such is also the case for the archaeocytes of sponge gemmules and the mesoblastic cells of phylactolaemate
statoblasts.
Furthermore, in all animal organization there are some cells which
retain to some degree an embryonic character, even while having a
specific orientation and histological fate—these are the replacement cells
to which names have been given such as histoblast and blastocyte. In
these, the functioning organism possesses the elements necessary to
compensate for death of cells and to repair tissues, worn by vital activities. Thus, these histoblasts are evidently designed to play a role in the
regeneration of the blastozooid. The histoblasts or blastocytes actually
are related to various tissues and have their particular histological
orientation. Herlant-Meewis (1964) notes their presence in ectoderm and
endoderm as well as mesoderm in Aeolosoma. The blastocysts of these
three layers become very active in the zone of scissiparity. Soon the
differentiated cells of the three layers become dedifferentiated and are
active in their turn. All these cells together form the three cell lines of the
blastema concerned with renewal of organogenesis. At this same level of
scissiparity, for structures related to the bud, the ectoblast derived from
ectoderm forms the head of the cerebral ganglia, the circumpharyngeal
connectives, the subpharyngeal ganglion, the bristle-bearing follicles of
the pharyngeal region, and the stomodaeum which unites with the neopharynx formed by the endoblast. The mesoblast forms the septa,
muscles, and nephridia of the anterior part of the blastozooid. In this
area of scissiparity, there is produced a remodeling of all the tissues
which resume their respective functions in the formation of a new zooid.