EARLY DEVELOPMENT OF THE SEA URCHIN
125
of the main embryological problem, that of the nuclear, viz. genetic,
control of morphogenesis and differentiation.
Although the data derived from work on sea urchins are not particularly abundant, there are nevertheless some important contributions
which deserve to be mentioned and discussed.
So far in the sea urchin, the influence of the nucleus on morphogenesis
cannot be demonstrated before the blastula stage or, more exactly, it is
at the blastula stage that some kind of nuclear intervention in morphogenesis has been indicated.
Apart from some enucleation experiments (Harvey, 1936 ; Lorch et al.,
1953; Hiramoto, 1956), which simply show that the removal of the
nucleus does not prevent cleavage though it results in abnormal
cytoplasmic masses, it is the hybridization experiments which deserve to
be mentioned first. It was shown by Baltzer (1910) that in the hybrid
combinations resulting in the elimination of the paternal chromosomes
during cleavage, development is invariably arrested at the blastula
stage. In some other combinations (Echinus QxAntedon
Sphaerechinus Ç χ Arbacia <$) no elimination occurs but nevertheless the developmental abnormality—the 'Erkrankung'—appears at the blastula
stage. In such cases the presence of both paternal and maternal
chromatin in the nuclei was ascertained. In some other crossings
(Echinus $ χ Arbacia $\ Paracentrotus
Ç χ Arbacia <$) the paternal
chromatin was eliminated as late as the blastula stage and the
'Erkrankung' became apparent almost at the same time.
Interesting data have been obtained by the study of some of these
hybrid combinations (Whiteley and Baltzer, 1958; Baltzer et al., 1958;
Baltzer and Chen, 1960; Chen et al., 1960). In the combination
Paracentrotus $ χ Arbacia
a distinct lowering of the respiratory rate
and a retardation of DNA synthesis became manifest coincident with
morphogenetic inhibition whereas no inhibition of RNA synthesis was
observed. On the other hand, in the combination Paracentrotus
$ χ
Sphaerechinus
<$ most of the paternal chromosomes are eliminated
during the first 3-4 cleavages and at the blastula stage they can be seen
scattered within the blastocoel. Such embryos never develop beyond the
stage of stereoblastula or stereogastrula. DNA determinations show
normal values for about 5 hours of development (about 32 cells) due to
the fact that the eliminated chromosomes continue to increase in
number for a few hours. At the time of hatching, however, the DNA
content of the hybrids is definitely lower and in the arrested embryos it
is only 50 to 62 per cent of that of the controls. Nevertheless, RNA
synthesis seems to proceed unimpaired at least for about 35 hours. This is
a point certainly worth further analysis. It would also be very interesting
to find out whether protein synthesis occurs at all in these embryos.
£
125
of the main embryological problem, that of the nuclear, viz. genetic,
control of morphogenesis and differentiation.
Although the data derived from work on sea urchins are not particularly abundant, there are nevertheless some important contributions
which deserve to be mentioned and discussed.
So far in the sea urchin, the influence of the nucleus on morphogenesis
cannot be demonstrated before the blastula stage or, more exactly, it is
at the blastula stage that some kind of nuclear intervention in morphogenesis has been indicated.
Apart from some enucleation experiments (Harvey, 1936 ; Lorch et al.,
1953; Hiramoto, 1956), which simply show that the removal of the
nucleus does not prevent cleavage though it results in abnormal
cytoplasmic masses, it is the hybridization experiments which deserve to
be mentioned first. It was shown by Baltzer (1910) that in the hybrid
combinations resulting in the elimination of the paternal chromosomes
during cleavage, development is invariably arrested at the blastula
stage. In some other combinations (Echinus QxAntedon
Sphaerechinus Ç χ Arbacia <$) no elimination occurs but nevertheless the developmental abnormality—the 'Erkrankung'—appears at the blastula
stage. In such cases the presence of both paternal and maternal
chromatin in the nuclei was ascertained. In some other crossings
(Echinus $ χ Arbacia $\ Paracentrotus
Ç χ Arbacia <$) the paternal
chromatin was eliminated as late as the blastula stage and the
'Erkrankung' became apparent almost at the same time.
Interesting data have been obtained by the study of some of these
hybrid combinations (Whiteley and Baltzer, 1958; Baltzer et al., 1958;
Baltzer and Chen, 1960; Chen et al., 1960). In the combination
Paracentrotus $ χ Arbacia
a distinct lowering of the respiratory rate
and a retardation of DNA synthesis became manifest coincident with
morphogenetic inhibition whereas no inhibition of RNA synthesis was
observed. On the other hand, in the combination Paracentrotus
$ χ
Sphaerechinus
<$ most of the paternal chromosomes are eliminated
during the first 3-4 cleavages and at the blastula stage they can be seen
scattered within the blastocoel. Such embryos never develop beyond the
stage of stereoblastula or stereogastrula. DNA determinations show
normal values for about 5 hours of development (about 32 cells) due to
the fact that the eliminated chromosomes continue to increase in
number for a few hours. At the time of hatching, however, the DNA
content of the hybrids is definitely lower and in the arrested embryos it
is only 50 to 62 per cent of that of the controls. Nevertheless, RNA
synthesis seems to proceed unimpaired at least for about 35 hours. This is
a point certainly worth further analysis. It would also be very interesting
to find out whether protein synthesis occurs at all in these embryos.
£