II.
THE
EMBRYOLOGY OF
ASCIDIANS
83
comparison with Fig. 37) and the larval characters (at least the palps) are
also not influenced by the foreign nucleus (matroclinous). The hybrid
larvae can enter metamorphosis (Figs. 38 to 40). As for the chromosomes,
they are always diploid; both paternal and maternal sets are present.
These results can be explained in two ways: either by assuming a
collaboration between the two nuclei and the cytoplasm, or by denying
any sort of collaboration by the foreign nucleus. The latter suggestion
is more probable. The possibility must not be excluded, however, of a
biochemical crisis occurring after metamorphosis.
4. Hybrid Andromerogons
No doubt the best means of studying the biochemical interrelationship
between the nucleus and cytoplasm is offered by hybrid andromerogons,
where the foreign nucleus is exposed to the egg's cytoplasm only. In
the Ascidians, only the combination Ascidia malaca ($) χ Phallusia
mamillata <$ has been studied (Minganti, 1959a). As mentioned above,
the diploid hybrids of this combination are viable and develop into
normal larvae; the homospermic andromerogons (haploid) are also
viable and give rise to normal larvae (Ortolani, 1958).
The removal of the egg nucleus is not difficult, as in A. malaca the
zone of the mitotic spindle is easily visible at the animal pole. With a
glass needle, one removes the entire animal pole; the non-nucleated egg
is then fertilized with foreign sperm. Two kinds of results are obtained:
lethal hybrid andromerogons, which are not even able to gastrulate and
viable hybrid andromerogons, a very small percentage of which are able to
develop to swimming larvae (Figs. 41 to 49). The first stages of development are normal in all the andromerogons, the egg dividing in every
detail as in A. malaca. The velocity of development, which differs in the
two species (in A. malaca, rapid; in Phallusia, slow), is that of A. malaca;
the character of the development is, consequently, matroclinous. This
fact is important as it reveals that the cyclic reversible modifications
which happen in the foreign nucleus, at every mitosis, are dictated by
the cytoplasm; possibly the cytoplasm produces cyclically a substance
(or substances) which influences the foreign nucleus. Thus we are
confronted here with a cytoplasmic 'determinant'; we shall discuss this
point later.
The lethal hybrid andromerogons, as stated above, stop developing at
the beginning of gastrulation; the morphogenetic movements do not
appear, and the embryos persist in the blástula stage for a long time
until they finally die. The cytological analysis reveals that the nuclei are
all quiescent in the globular state; no pycnotic nuclei, or chromatin
elimination (as found in the diploid hybrids) can be detected. It seems
as if the cytoplasmic substance dictating the rhythm of mitotic transit
A.M. I
THE
EMBRYOLOGY OF
ASCIDIANS
83
comparison with Fig. 37) and the larval characters (at least the palps) are
also not influenced by the foreign nucleus (matroclinous). The hybrid
larvae can enter metamorphosis (Figs. 38 to 40). As for the chromosomes,
they are always diploid; both paternal and maternal sets are present.
These results can be explained in two ways: either by assuming a
collaboration between the two nuclei and the cytoplasm, or by denying
any sort of collaboration by the foreign nucleus. The latter suggestion
is more probable. The possibility must not be excluded, however, of a
biochemical crisis occurring after metamorphosis.
4. Hybrid Andromerogons
No doubt the best means of studying the biochemical interrelationship
between the nucleus and cytoplasm is offered by hybrid andromerogons,
where the foreign nucleus is exposed to the egg's cytoplasm only. In
the Ascidians, only the combination Ascidia malaca ($) χ Phallusia
mamillata <$ has been studied (Minganti, 1959a). As mentioned above,
the diploid hybrids of this combination are viable and develop into
normal larvae; the homospermic andromerogons (haploid) are also
viable and give rise to normal larvae (Ortolani, 1958).
The removal of the egg nucleus is not difficult, as in A. malaca the
zone of the mitotic spindle is easily visible at the animal pole. With a
glass needle, one removes the entire animal pole; the non-nucleated egg
is then fertilized with foreign sperm. Two kinds of results are obtained:
lethal hybrid andromerogons, which are not even able to gastrulate and
viable hybrid andromerogons, a very small percentage of which are able to
develop to swimming larvae (Figs. 41 to 49). The first stages of development are normal in all the andromerogons, the egg dividing in every
detail as in A. malaca. The velocity of development, which differs in the
two species (in A. malaca, rapid; in Phallusia, slow), is that of A. malaca;
the character of the development is, consequently, matroclinous. This
fact is important as it reveals that the cyclic reversible modifications
which happen in the foreign nucleus, at every mitosis, are dictated by
the cytoplasm; possibly the cytoplasm produces cyclically a substance
(or substances) which influences the foreign nucleus. Thus we are
confronted here with a cytoplasmic 'determinant'; we shall discuss this
point later.
The lethal hybrid andromerogons, as stated above, stop developing at
the beginning of gastrulation; the morphogenetic movements do not
appear, and the embryos persist in the blástula stage for a long time
until they finally die. The cytological analysis reveals that the nuclei are
all quiescent in the globular state; no pycnotic nuclei, or chromatin
elimination (as found in the diploid hybrids) can be detected. It seems
as if the cytoplasmic substance dictating the rhythm of mitotic transit
A.M. I