324
T. A. DETTLAFF
developmental mechanics, in connection with the Weismann-Roux
hypothesis of the hereditarily unequal division of the nucleus during
cleavage. Later, because of the many facts contradicting it, this hypothesis
was abandoned, and it was the problems of the degree of differentiation
of the cytoplasm in the eggs of different animals, of the distribution of
qualitatively different cytoplasmic components during cleavage (cytoplasm segregation), and of their morphogenetic role which moved to the
foreground.
This stage of investigation resulted in the concept that different
substances of the egg cytoplasm, coming to lie in different blastomeres
during cleavage, affected the activity of different genes in the nuclei
that, in their turn, affected the cytoplasm, bringing about new interactions leading to a gradual differentiation of the parts of the embryo
(Morgan, 1934).
In its general form this concept is accepted nowadays (cf. Waddington,
1950; Brächet, 1960; Neyfakh, 1961, Lopashov, 1963) being supported
by the data accumulated during recent decades. The segregation of the
cytoplasm is not questioned; the segregation of individual inclusions
and biochemical components of the cytoplasm has been studied in many
animal species although no morphogenetically active substances have
been identified as yet (cf. Lehmann, 1945; Raven, 1958; Brächet, 1957,
1960; Weber, 1960). Experiments on nuclear transplantation have given
new clear evidence of the equipotentiality of nuclei during cleavage
(Briggs and King, 1953; cf. Briggs and King, 1959). Finally, there are a
few experiments revealing the differentiating action of the cytoplasmic
segregation upon nuclear function, and vice versa, the effect of the
nuclei, after the onset of their function, upon the differentiation of the
cytoplasm (cf. Brächet, 1957, 1960; Briggs and King, 1959).
As to the question of the distribution of these processes in time, and
as to when the simple segregation of the protoplasm is replaced by the
synthesis of specific cytoplasmic proteins, the erroneous idea is widely
accepted in the literature that this synthesis starts with the onset of
gastrulation. The gastrulation period is thus opposed to that of cleavage.
The latter, however, is heterogeneous, including not only a period of
synchronous divisions, but also a period of asynchronous divisions (the
stages of mid and late blastula). Costello (1955) distinguished this
asynchronous period as 'blastulation'; this term, however, is seldom
used. The asynchronous period was found in several experiments to be
characterized by an activity of the nuclei (cf. II).
On the other hand, data are available in the literature which show
that in closely related animal groups differences in the degree of latent
differentiation of rudiments at the same developmental stage correspond
to differences in the number of cell divisions that have taken place up to
T. A. DETTLAFF
developmental mechanics, in connection with the Weismann-Roux
hypothesis of the hereditarily unequal division of the nucleus during
cleavage. Later, because of the many facts contradicting it, this hypothesis
was abandoned, and it was the problems of the degree of differentiation
of the cytoplasm in the eggs of different animals, of the distribution of
qualitatively different cytoplasmic components during cleavage (cytoplasm segregation), and of their morphogenetic role which moved to the
foreground.
This stage of investigation resulted in the concept that different
substances of the egg cytoplasm, coming to lie in different blastomeres
during cleavage, affected the activity of different genes in the nuclei
that, in their turn, affected the cytoplasm, bringing about new interactions leading to a gradual differentiation of the parts of the embryo
(Morgan, 1934).
In its general form this concept is accepted nowadays (cf. Waddington,
1950; Brächet, 1960; Neyfakh, 1961, Lopashov, 1963) being supported
by the data accumulated during recent decades. The segregation of the
cytoplasm is not questioned; the segregation of individual inclusions
and biochemical components of the cytoplasm has been studied in many
animal species although no morphogenetically active substances have
been identified as yet (cf. Lehmann, 1945; Raven, 1958; Brächet, 1957,
1960; Weber, 1960). Experiments on nuclear transplantation have given
new clear evidence of the equipotentiality of nuclei during cleavage
(Briggs and King, 1953; cf. Briggs and King, 1959). Finally, there are a
few experiments revealing the differentiating action of the cytoplasmic
segregation upon nuclear function, and vice versa, the effect of the
nuclei, after the onset of their function, upon the differentiation of the
cytoplasm (cf. Brächet, 1957, 1960; Briggs and King, 1959).
As to the question of the distribution of these processes in time, and
as to when the simple segregation of the protoplasm is replaced by the
synthesis of specific cytoplasmic proteins, the erroneous idea is widely
accepted in the literature that this synthesis starts with the onset of
gastrulation. The gastrulation period is thus opposed to that of cleavage.
The latter, however, is heterogeneous, including not only a period of
synchronous divisions, but also a period of asynchronous divisions (the
stages of mid and late blastula). Costello (1955) distinguished this
asynchronous period as 'blastulation'; this term, however, is seldom
used. The asynchronous period was found in several experiments to be
characterized by an activity of the nuclei (cf. II).
On the other hand, data are available in the literature which show
that in closely related animal groups differences in the degree of latent
differentiation of rudiments at the same developmental stage correspond
to differences in the number of cell divisions that have taken place up to