112
ALBERTO MONROY AND RACHELE MAGGIO
and of cell division. The significance of this plateau is obscure. However,
it is just at the beginning of the plateau that the first visible differentiation in the sea urchin embryo occurs, namely the formation of the cilia.
The antagonism between differentiation and cell division has long been
known. Whether the coincidence between the plateau on the one hand
and the appearance of cilia on the other is purely fortuitous, or whether
a causal link exists between the two phenomena, is at present hard to
say, but it is certainly worth looking into (see also Stockdale and
Holtzer, 1961). No doubt the formation of cilia requires the presence of
the specific fibrous proteins typical of these structures. Whether these
proteins are actually synthesized at the time of the formation of the cilia
or are already present in the egg and simply put together, is impossible to
decide. Indeed, no chemical or serological evidence whatsoever is available at the moment on this point, nor is anything known about the
structure and the mechanism of the formation of the cilia. Actually,
none of the isotope work quoted in the previous section seems to suggest
any change in the rate of incorporation in conjunction with ciliation.
Only the experiments of Hoberman et al. (1952) in which Arbacia eggs
were raised in the presence of heavy water show that, while, during the
first few hours following fertilization incorporation proceeds almost at
the same rate as in the unfertilized eggs, at the beginning of the blastula
stage it undergoes a sudden and marked increase.
A new phase in the development of the sea urchin embryo comes into
being at the time of the migration of the cells of the primary mesenchyme.
This phase coincides with the new increase in oxygen consumption and
studies with labelled compounds have indicated that at this stage also
their rate of incorporation into proteins undergoes a new increase. The
serological experiments (Perlmann and Gustafson, 1948; Perlmann,
1953; Ranzi, 1957, 1960; Ishida and Yasumasu, 1957) indicate that
during this second phase of development new proteins begin to be
synthesized in measurable quantities and adult-type proteins make their
first appearance (Fig. 10). However, the serological tests are so far the
only clear-cut evidence of this. Electrophoretic analyses of the total
soluble proteins as well as fractionation on cellulose column (DEAE)
have so far failed to show any qualitative difference between unfertilized
eggs and stages as advanced as the prism (Monroy et al., 1961b). This
failure may depend on the lower resolving power of these methods as
compared to the high sensitivity of the serological methods. Recent
experiments in which use was made of the new disc electrophoretic
technique (Pfohl and Monroy, 1962) have also shown a substantially
identical pattern of components throughout development, except for
(a) an esterase band which from the mesenchyme blastula onwards
appears to be split into two bands, and (b) the presence from the same
ALBERTO MONROY AND RACHELE MAGGIO
and of cell division. The significance of this plateau is obscure. However,
it is just at the beginning of the plateau that the first visible differentiation in the sea urchin embryo occurs, namely the formation of the cilia.
The antagonism between differentiation and cell division has long been
known. Whether the coincidence between the plateau on the one hand
and the appearance of cilia on the other is purely fortuitous, or whether
a causal link exists between the two phenomena, is at present hard to
say, but it is certainly worth looking into (see also Stockdale and
Holtzer, 1961). No doubt the formation of cilia requires the presence of
the specific fibrous proteins typical of these structures. Whether these
proteins are actually synthesized at the time of the formation of the cilia
or are already present in the egg and simply put together, is impossible to
decide. Indeed, no chemical or serological evidence whatsoever is available at the moment on this point, nor is anything known about the
structure and the mechanism of the formation of the cilia. Actually,
none of the isotope work quoted in the previous section seems to suggest
any change in the rate of incorporation in conjunction with ciliation.
Only the experiments of Hoberman et al. (1952) in which Arbacia eggs
were raised in the presence of heavy water show that, while, during the
first few hours following fertilization incorporation proceeds almost at
the same rate as in the unfertilized eggs, at the beginning of the blastula
stage it undergoes a sudden and marked increase.
A new phase in the development of the sea urchin embryo comes into
being at the time of the migration of the cells of the primary mesenchyme.
This phase coincides with the new increase in oxygen consumption and
studies with labelled compounds have indicated that at this stage also
their rate of incorporation into proteins undergoes a new increase. The
serological experiments (Perlmann and Gustafson, 1948; Perlmann,
1953; Ranzi, 1957, 1960; Ishida and Yasumasu, 1957) indicate that
during this second phase of development new proteins begin to be
synthesized in measurable quantities and adult-type proteins make their
first appearance (Fig. 10). However, the serological tests are so far the
only clear-cut evidence of this. Electrophoretic analyses of the total
soluble proteins as well as fractionation on cellulose column (DEAE)
have so far failed to show any qualitative difference between unfertilized
eggs and stages as advanced as the prism (Monroy et al., 1961b). This
failure may depend on the lower resolving power of these methods as
compared to the high sensitivity of the serological methods. Recent
experiments in which use was made of the new disc electrophoretic
technique (Pfohl and Monroy, 1962) have also shown a substantially
identical pattern of components throughout development, except for
(a) an esterase band which from the mesenchyme blastula onwards
appears to be split into two bands, and (b) the presence from the same