62
G. RE VERBERI
that Feulgen-positive granules are also present in the cytoplasm of the
ascidian Molgula; this is certainly not the case in the cytoplasm of the
Ciona egg. In the nuclei of the first blastomeres, the Feulgen reaction is
very weak, and no differences in staining intensity have been discovered.
The reason for the weakness of the reaction is not due to scarcity of
DNA, but rather to the fact that it is very dilute; in fact, in vivo, the
nuclei of the first blastomeres are transparent and collapse very easily
if one tries to isolate them from the cytoplasm with a micro-pipette
(e.g. to transplant them). The nuclei of the gastrula stage show, on the
contrary, a strong Feulgen reaction; furthermore, they show differences
in the intensity of coloration, the nuclei of the nervous and mesenchymal
cells appearing more deeply coloured than the nuclei of the other cells.
The nuclei of the more advanced stages do not present any other
difference. The strong coloration of the nuclei of the nervous and
mesenchymal cells in the gastrula stage is not necessarily due to a
higher content of DNA, but, more probably, to the fact that the nuclei
are more condensed. Measurements of the volumes of the nuclei of
different tissues show that the nuclei of the muscular cells are very
large while the nuclei of the nervous and mesenchymal cells, on the
contrary, are small. The theory of the constancy of the content of DNA
is not contradicted by these facts. On the other hand the assumption by
Dalcq (1957b) that the cells which display a strong mitotic activity are
also rich in DNA, is supported by the behaviour of the nuclei of the
neural and mesenchymal cells.
3. Glycogen
The fate of glycogen during development has been also followed by
Mancuso (1956). As already noted, the granules of glycogen migrate
after fertilization to the surface of the egg; the areas of the yellow and
grey crescent, however, lack them completely. At the 8-cell stage, the
glycogen is particularly abundant in the entodermal plasm of the
vegetal blastomeres but is absent, however, in the meso- and chordoplasm. Very small quantities of glycogen are also present in the animal
blastomeres. At the gastrula stage, only the entodermal cells appear to
be rich in glycogen; chordal or mesodermal cells, which form, respectively, the dorsal and the lateral lips of the blastopore, do not reveal
the presence of any glycogen. These, in fact, deriva respectively from
the yellow and from the grey plasm. One can thus see how different the
situation is from that in the Amphibians, where the dorsal lip of the
blastopore, or chorda-mesoderm, is rich in glycogen, which is partly
consumed during the movements of gastrulation (Heatley, 1935;
Jaeger, 1945).
At the tail-bud stage and in the swimming larva the intestine and the
G. RE VERBERI
that Feulgen-positive granules are also present in the cytoplasm of the
ascidian Molgula; this is certainly not the case in the cytoplasm of the
Ciona egg. In the nuclei of the first blastomeres, the Feulgen reaction is
very weak, and no differences in staining intensity have been discovered.
The reason for the weakness of the reaction is not due to scarcity of
DNA, but rather to the fact that it is very dilute; in fact, in vivo, the
nuclei of the first blastomeres are transparent and collapse very easily
if one tries to isolate them from the cytoplasm with a micro-pipette
(e.g. to transplant them). The nuclei of the gastrula stage show, on the
contrary, a strong Feulgen reaction; furthermore, they show differences
in the intensity of coloration, the nuclei of the nervous and mesenchymal
cells appearing more deeply coloured than the nuclei of the other cells.
The nuclei of the more advanced stages do not present any other
difference. The strong coloration of the nuclei of the nervous and
mesenchymal cells in the gastrula stage is not necessarily due to a
higher content of DNA, but, more probably, to the fact that the nuclei
are more condensed. Measurements of the volumes of the nuclei of
different tissues show that the nuclei of the muscular cells are very
large while the nuclei of the nervous and mesenchymal cells, on the
contrary, are small. The theory of the constancy of the content of DNA
is not contradicted by these facts. On the other hand the assumption by
Dalcq (1957b) that the cells which display a strong mitotic activity are
also rich in DNA, is supported by the behaviour of the nuclei of the
neural and mesenchymal cells.
3. Glycogen
The fate of glycogen during development has been also followed by
Mancuso (1956). As already noted, the granules of glycogen migrate
after fertilization to the surface of the egg; the areas of the yellow and
grey crescent, however, lack them completely. At the 8-cell stage, the
glycogen is particularly abundant in the entodermal plasm of the
vegetal blastomeres but is absent, however, in the meso- and chordoplasm. Very small quantities of glycogen are also present in the animal
blastomeres. At the gastrula stage, only the entodermal cells appear to
be rich in glycogen; chordal or mesodermal cells, which form, respectively, the dorsal and the lateral lips of the blastopore, do not reveal
the presence of any glycogen. These, in fact, deriva respectively from
the yellow and from the grey plasm. One can thus see how different the
situation is from that in the Amphibians, where the dorsal lip of the
blastopore, or chorda-mesoderm, is rich in glycogen, which is partly
consumed during the movements of gastrulation (Heatley, 1935;
Jaeger, 1945).
At the tail-bud stage and in the swimming larva the intestine and the