II. THE EMBRYOLOGY OF
ASCIDIANS
91
of their membranes. These experiments, which were carried out some
time ago (Reverberi, 1940; Almagia, 1946) on Ciona and Ascidia malaca,
gave results which could not be properly interpreted. With high-speed
centrifugation the plasms separate, and the egg breaks into two or more
fragments; 'dark' and 'hyaline' fragments are obtained, the former
containing yolk granules, pigments, and mitochondria, and the latter
consisting of transparent plasm. After fertilization, in Ciona, only the
'dark' fragments developed, the 'hyaline' did not segment; in Ascidia
malaca, however, both fragments, the 'dark' and the 'hyaline', segmented
and developed; in any case, neither in Ciona nor in A. malaca were
swimming tadpoles obtained. These results are a contradiction of the
assumption that the eggs are totipotent.
In recent work (Reverberi and La Spina, 1959) in Ciona only those experiments were taken into consideration the controls of which developed
100% normally. Care was also taken to use a refrigerated centrifuge. The
results were as follows: the 'hyaline' fragments do not develop; they try
to segment, presenting continuous ameboid modifications, but after a
long and useless activity, they undergo lysis. One might impute the
failure of these fragments to the haploid state (as they possess only the
sperm nucleus), but the experiments on merogony mentioned above
(Ortolani, 1958) establish that haploid tadpoles can be completely
normal. Also the large size of the fragments cannot be blamed, as 'dark*
fragments of the same size develop. It is very strange that the hyaline
fragments, which are of pure 'ground substance' or endoplasmic reticulum with much RNA and glycogen, are not even capable of segmentation! Since they do not possess mitochondria, one could attribute
their lack of development to this.
As regards the 'dark' fragments, they segment like an egg in miniature;
the segmentation pattern is typical, the gastrulation is normal, and
finally they give rise to normal, swimming larvae. This result was never
obtained in the past. The 'dark' fragments possess all the egg components ; they are deprived of hyaline plasm only by the centrifugation.
One cannot escape the conclusion that the hyaline plasm, at least at
this stage of the development, is lacking in every Organ-forming'
substance. If it were a mosaic of Organ-forming' potencies, it would not
be possible to remove such a large portion of it with no effect on
morphogenesis.
In Ascidia malaca, as already mentioned, both fragments are able to
develop. The explanation of this is given by the centrifugation experiments of Urbani and Urbani-Mistruzzi (1947). In A. malaca, in contrast
to Ciona, the mitochondria are distributed in two areas of the egg;
there are 'light' mitochondria, which segregate in the 'dark' fragment, and
'heavy' mitochondria which segregate in the 'hyaline' fragment. If the
ASCIDIANS
91
of their membranes. These experiments, which were carried out some
time ago (Reverberi, 1940; Almagia, 1946) on Ciona and Ascidia malaca,
gave results which could not be properly interpreted. With high-speed
centrifugation the plasms separate, and the egg breaks into two or more
fragments; 'dark' and 'hyaline' fragments are obtained, the former
containing yolk granules, pigments, and mitochondria, and the latter
consisting of transparent plasm. After fertilization, in Ciona, only the
'dark' fragments developed, the 'hyaline' did not segment; in Ascidia
malaca, however, both fragments, the 'dark' and the 'hyaline', segmented
and developed; in any case, neither in Ciona nor in A. malaca were
swimming tadpoles obtained. These results are a contradiction of the
assumption that the eggs are totipotent.
In recent work (Reverberi and La Spina, 1959) in Ciona only those experiments were taken into consideration the controls of which developed
100% normally. Care was also taken to use a refrigerated centrifuge. The
results were as follows: the 'hyaline' fragments do not develop; they try
to segment, presenting continuous ameboid modifications, but after a
long and useless activity, they undergo lysis. One might impute the
failure of these fragments to the haploid state (as they possess only the
sperm nucleus), but the experiments on merogony mentioned above
(Ortolani, 1958) establish that haploid tadpoles can be completely
normal. Also the large size of the fragments cannot be blamed, as 'dark*
fragments of the same size develop. It is very strange that the hyaline
fragments, which are of pure 'ground substance' or endoplasmic reticulum with much RNA and glycogen, are not even capable of segmentation! Since they do not possess mitochondria, one could attribute
their lack of development to this.
As regards the 'dark' fragments, they segment like an egg in miniature;
the segmentation pattern is typical, the gastrulation is normal, and
finally they give rise to normal, swimming larvae. This result was never
obtained in the past. The 'dark' fragments possess all the egg components ; they are deprived of hyaline plasm only by the centrifugation.
One cannot escape the conclusion that the hyaline plasm, at least at
this stage of the development, is lacking in every Organ-forming'
substance. If it were a mosaic of Organ-forming' potencies, it would not
be possible to remove such a large portion of it with no effect on
morphogenesis.
In Ascidia malaca, as already mentioned, both fragments are able to
develop. The explanation of this is given by the centrifugation experiments of Urbani and Urbani-Mistruzzi (1947). In A. malaca, in contrast
to Ciona, the mitochondria are distributed in two areas of the egg;
there are 'light' mitochondria, which segregate in the 'dark' fragment, and
'heavy' mitochondria which segregate in the 'hyaline' fragment. If the