200
B. L. ASTAUROV
I. Introduction and Brief Historical Survey
A. Introductory Remarks
The alterations of the cytogenetic mechanisms of development that we
shall later deal with in this review are confined mostly to the periods of
maturation divisions of the oocyte, to fertilization, and to early cleavage
of the naturally or artificially activated egg.
These alterations are various changes in the normal order of nuclear
movements and combinations and, as a consequence, in the reorganization of the chromosome constitution (that is, of the genotype) of the
future embryos, which occur in the course of the spontaneous initiation
or artificial induction of parthenogenesis, gynogenesis, and androgenesis,
or as a result of natural and experimental polyploidization. The normal
course of maturation, fertilization, and early embryogenesis must also
be touched upon as the necessary background. Only systemic changes
of genotype will be discussed. The morphogenetic significance of gene
mutations, both spontaneous and artificial, is beyond the scope of this
review.
These phenomena usually pertain to the domain of experimental
cytology and cytogenetics, and are studied from the point of view of
cell physiology (mechanisms of activation and cell division) and genetics
(mechanisms of hereditary transmission, meiosis, formation of the
zygote), rather than from that of morphogenesis. Although all these
above-mentioned cytogenetic events occur at the moment traditionally
regarded as the starting point of development, with respect to morphogenesis the process of development begins much earlier.
Many observations, including perhaps most convincingly experiments
on the so-called parthenogenetic merogony (Harvey, 1936, 1939), have
shown that some important steps in the causal sequence of formative
processes are already passed during preembryonic stages of ontogenesis.
It is the promorphological cytoplasmic organization of the egg, roughly
shaped in the course of oogenesis under the control of the maternal
genetic constitution, which alone predetermines and ensures the fundamental traits and the specificity of early embryogenesis. Therefore, the
transformation of the maternal genotype of preembryonic stages into
that of the future embryo at the conventional starting point of development plays but a minor morphogenetic role. All that can be said about
the transformation of the genetic constitution in the course of normal
maturation and fertilization holds true for its change under the abovementioned experimental alterations of the cytological mechanisms of
development.
As will be seen later, a surprisingly great number of rather drastic
B. L. ASTAUROV
I. Introduction and Brief Historical Survey
A. Introductory Remarks
The alterations of the cytogenetic mechanisms of development that we
shall later deal with in this review are confined mostly to the periods of
maturation divisions of the oocyte, to fertilization, and to early cleavage
of the naturally or artificially activated egg.
These alterations are various changes in the normal order of nuclear
movements and combinations and, as a consequence, in the reorganization of the chromosome constitution (that is, of the genotype) of the
future embryos, which occur in the course of the spontaneous initiation
or artificial induction of parthenogenesis, gynogenesis, and androgenesis,
or as a result of natural and experimental polyploidization. The normal
course of maturation, fertilization, and early embryogenesis must also
be touched upon as the necessary background. Only systemic changes
of genotype will be discussed. The morphogenetic significance of gene
mutations, both spontaneous and artificial, is beyond the scope of this
review.
These phenomena usually pertain to the domain of experimental
cytology and cytogenetics, and are studied from the point of view of
cell physiology (mechanisms of activation and cell division) and genetics
(mechanisms of hereditary transmission, meiosis, formation of the
zygote), rather than from that of morphogenesis. Although all these
above-mentioned cytogenetic events occur at the moment traditionally
regarded as the starting point of development, with respect to morphogenesis the process of development begins much earlier.
Many observations, including perhaps most convincingly experiments
on the so-called parthenogenetic merogony (Harvey, 1936, 1939), have
shown that some important steps in the causal sequence of formative
processes are already passed during preembryonic stages of ontogenesis.
It is the promorphological cytoplasmic organization of the egg, roughly
shaped in the course of oogenesis under the control of the maternal
genetic constitution, which alone predetermines and ensures the fundamental traits and the specificity of early embryogenesis. Therefore, the
transformation of the maternal genotype of preembryonic stages into
that of the future embryo at the conventional starting point of development plays but a minor morphogenetic role. All that can be said about
the transformation of the genetic constitution in the course of normal
maturation and fertilization holds true for its change under the abovementioned experimental alterations of the cytological mechanisms of
development.
As will be seen later, a surprisingly great number of rather drastic