CELL DIVISIONS
345
embryos consist of a smaller number of cells belonging to younger
generations than those in the axolotl. At the same developmental
stages frog embryos consist of a greater number of cells of older
generations than those of the axolotl. Finally, sturgeon and sevruga
embryos at the same stages contain cells of the same cell generations.
Correspondingly, the total duration of interkinetic states of nuclei at the
onset of the same developmental stages is in newts shorter than in the
axolotl, and in axolotl shorter than in frogs, and on the contrary, this
duration is similar in sturgeons and sevruga.
In order to judge the morphogenetic role of the differences in the
relationships of cleavage, gastrulation and neurulation, data will be
discussed in the next section on the time of onset of determination and
the degree of determination of rudiments at the same developmental
stage (a) in embryos of one species when the relationships of cell
generations and developmental stages are changed by sublethal
temperatures, Li ions, or temporary isolation of the embryonic material
in physiological solution, and (b) in the embryos of different species of
amphibians and sturgeon fishes.
V· Morphogenetic Role of the Differences in the Ratio between
the Durations of Cleavage, Gastrulation and Neurulation
One of the criteria of the process of chemical differentiation underlying
morphogenesis (this process was called 'intrinsic development' by Ten
Cate, 'latent differentiation' by Dragomirov) which is accessible to
experimental study is the time of onset and the degree of determination
of the rudiments at successive developmental stages. During the last
30-40 years much work in experimental embryology has been of this
kind. During the recent decade the central place has been taken by a
penetrating cytochemical, immunological, and electron microscopical
study of the processes of differentiation. Along with the investigation of
the interaction between nucleus and cytoplasm, these researches have
brought about great progress in modern embryology and open up still
greater vistas for the near future. The old data on the determination
processes have been supplied with a new physiological essence. However,
even without this, they are of great importance because they give us an
opportunity to judge both the rate and co-ordination of the changes in
the regions of the embryo during its development, and the significance
of differences in the interrelationship of individual developmental
periods. Applied in this way (Filatov, 1934, 1937, 1939a, b, 1941a, b,
1943; Ginsburg, 1946b, c, 1950a, b; Schmalhausen, 1938; Ten Cate,
1956; Dettlaff, 1956; Lopashov, 1963; Chulitskaya, 1961b, 1962, and
others), they have revealed important generalizations that are in part
discussed below.
345
embryos consist of a smaller number of cells belonging to younger
generations than those in the axolotl. At the same developmental
stages frog embryos consist of a greater number of cells of older
generations than those of the axolotl. Finally, sturgeon and sevruga
embryos at the same stages contain cells of the same cell generations.
Correspondingly, the total duration of interkinetic states of nuclei at the
onset of the same developmental stages is in newts shorter than in the
axolotl, and in axolotl shorter than in frogs, and on the contrary, this
duration is similar in sturgeons and sevruga.
In order to judge the morphogenetic role of the differences in the
relationships of cleavage, gastrulation and neurulation, data will be
discussed in the next section on the time of onset of determination and
the degree of determination of rudiments at the same developmental
stage (a) in embryos of one species when the relationships of cell
generations and developmental stages are changed by sublethal
temperatures, Li ions, or temporary isolation of the embryonic material
in physiological solution, and (b) in the embryos of different species of
amphibians and sturgeon fishes.
V· Morphogenetic Role of the Differences in the Ratio between
the Durations of Cleavage, Gastrulation and Neurulation
One of the criteria of the process of chemical differentiation underlying
morphogenesis (this process was called 'intrinsic development' by Ten
Cate, 'latent differentiation' by Dragomirov) which is accessible to
experimental study is the time of onset and the degree of determination
of the rudiments at successive developmental stages. During the last
30-40 years much work in experimental embryology has been of this
kind. During the recent decade the central place has been taken by a
penetrating cytochemical, immunological, and electron microscopical
study of the processes of differentiation. Along with the investigation of
the interaction between nucleus and cytoplasm, these researches have
brought about great progress in modern embryology and open up still
greater vistas for the near future. The old data on the determination
processes have been supplied with a new physiological essence. However,
even without this, they are of great importance because they give us an
opportunity to judge both the rate and co-ordination of the changes in
the regions of the embryo during its development, and the significance
of differences in the interrelationship of individual developmental
periods. Applied in this way (Filatov, 1934, 1937, 1939a, b, 1941a, b,
1943; Ginsburg, 1946b, c, 1950a, b; Schmalhausen, 1938; Ten Cate,
1956; Dettlaff, 1956; Lopashov, 1963; Chulitskaya, 1961b, 1962, and
others), they have revealed important generalizations that are in part
discussed below.