90
G.
REVERBERI
Ortolani) that every fragment of the unsegmented egg is able to give
rise to a normal, although smaller, tadpole; a reservation must be made
for a small area at the animal pole, which is not able to divide with the
typical pattern, or to give rise to more than one blástula. This can be
explained by the fact that this zone is deprived of mitochondria to
which, in this paper, has been attributed such a strong influence on
morphogenesis.
The centrifugation of the unfertilized Ascidian egg was carried out by
Ries (1939) and by Tung et al. (1941). Tung et al. concluded from the
results of their experiments with Ciona eggs, where they obtained
'chaotic' embryos, that the Ascidian egg, even in the unsegmented
stage, is a 'mosaic', or a 'precociously determined' egg: the visible or
ground-substances would, then, be responsible, from the very beginning,
for larval structures. Ries (1937), also, came to a similar conclusion.
In fact he maintains that if, by centrifugation, one displaces the 'plasm'
which is rich in 'indophenol oxidases' (checked by the Nadi-reaction),
the larval muscles are also displaced correspondingly; the 'myoplasm'
would thus be, according to Ries, a true 'muscle-forming' substance.
However, Ries was misled by supposing that the 'indophenol oxidases'
are bound to the hyaline plasm; on the contrary, they are situated in the
mitochondria (Reverberi, 1957a); the results of Ries then, demonstrate
only that the mitochondria (as shown by Reverberi, 1957a, b) are very
important for differentiation of the musculature (cf. also Brächet, 1957,
p. 387).
The experiments of La Spina (1958) mentioned above gave results
which are completely contrary to the assumption that the unsegmented
egg is already a mosaic of 'organ-forming substances'. From the
unfertilized centrifuged eggs, larvae were obtained which looked
completely, or nearly, normal; only a small percentage of 'chaotic'
embryos were obtained. As Ries had done earlier, La Spina also followed
the displacement of the mitochondria using the Nadi-reaction; very
often the plasms, in the intervals between centrifugation, fertilization
and segmentation, were more or less redistributed. Special care was
taken with the controls, the only experiments considered being those
where 95-100% of the controls developed completely normally;
moreover eggs which segmented abnormally were discarded. With the
use of these precautions it became clear why Tung et al. obtained
'chaotic' larvae: it seems, in fact, that the authors chose only the eggs
which presented from the beginning an anomalous segmentation and
gastrulation. In the Ascidians, however, every egg (not centrifuged)
which segments abnormally, always gives rise to chaotic embryos.
More definite results were obtained (Reverberi and La Spina, 1959;
La Spina, 1959) by the centrifugation of 'naked' eggs, or eggs deprived
G.
REVERBERI
Ortolani) that every fragment of the unsegmented egg is able to give
rise to a normal, although smaller, tadpole; a reservation must be made
for a small area at the animal pole, which is not able to divide with the
typical pattern, or to give rise to more than one blástula. This can be
explained by the fact that this zone is deprived of mitochondria to
which, in this paper, has been attributed such a strong influence on
morphogenesis.
The centrifugation of the unfertilized Ascidian egg was carried out by
Ries (1939) and by Tung et al. (1941). Tung et al. concluded from the
results of their experiments with Ciona eggs, where they obtained
'chaotic' embryos, that the Ascidian egg, even in the unsegmented
stage, is a 'mosaic', or a 'precociously determined' egg: the visible or
ground-substances would, then, be responsible, from the very beginning,
for larval structures. Ries (1937), also, came to a similar conclusion.
In fact he maintains that if, by centrifugation, one displaces the 'plasm'
which is rich in 'indophenol oxidases' (checked by the Nadi-reaction),
the larval muscles are also displaced correspondingly; the 'myoplasm'
would thus be, according to Ries, a true 'muscle-forming' substance.
However, Ries was misled by supposing that the 'indophenol oxidases'
are bound to the hyaline plasm; on the contrary, they are situated in the
mitochondria (Reverberi, 1957a); the results of Ries then, demonstrate
only that the mitochondria (as shown by Reverberi, 1957a, b) are very
important for differentiation of the musculature (cf. also Brächet, 1957,
p. 387).
The experiments of La Spina (1958) mentioned above gave results
which are completely contrary to the assumption that the unsegmented
egg is already a mosaic of 'organ-forming substances'. From the
unfertilized centrifuged eggs, larvae were obtained which looked
completely, or nearly, normal; only a small percentage of 'chaotic'
embryos were obtained. As Ries had done earlier, La Spina also followed
the displacement of the mitochondria using the Nadi-reaction; very
often the plasms, in the intervals between centrifugation, fertilization
and segmentation, were more or less redistributed. Special care was
taken with the controls, the only experiments considered being those
where 95-100% of the controls developed completely normally;
moreover eggs which segmented abnormally were discarded. With the
use of these precautions it became clear why Tung et al. obtained
'chaotic' larvae: it seems, in fact, that the authors chose only the eggs
which presented from the beginning an anomalous segmentation and
gastrulation. In the Ascidians, however, every egg (not centrifuged)
which segments abnormally, always gives rise to chaotic embryos.
More definite results were obtained (Reverberi and La Spina, 1959;
La Spina, 1959) by the centrifugation of 'naked' eggs, or eggs deprived