252
B. L. ASTAUROV
different ways of development; the normal process of reproduction is
included.
It is also worth mentioning that each cytogenetic mechanism realized
so far can work with two different physiological modifications of embryonic development, i.e., with or without diapause.
The diapause (or alternatively nondiapause) character of the silkworm
embryo depends on the maternal genetic constitution as well as on
environmental conditions, especially on light and temperature during
the sensitive period comprising the second half of the embryonic development of the mother. Finally, both genetic and environmental factors,
when determining the diapause or nondiapause development, operate
through the neurohormonal regulatory mechanism. Although very important from the standpoint of physiological genetics, this developmental
problem lies beyond the scope of this review (cf. the literature in Astaurov, 1933; Morohoshi, 1957; Tazima, 1964).
A certain degree of ploidy can be obtained and maintained on the
basis of different cytogenetic mechanisms. For instance, the diploid constitution can be achieved in six different ways (Table III, A2, B2, C2,
D2, E2, and F2). Vice versa, the same cytogenetic mechanism could
successfully work with various numbers of chromosome sets, for example,
the ameiotic mode of oocyte maturation in cases of di-, tri-, and tetraploid thermal parthenogenesis (Table III, F2, F3, and F4).
The alterations already obtained and consistent with successful development up to the imago stage are marked in Table III with "-|—f-";
those which can and do arise but are unviable are denoted with " - ) — " ;
references to chapters and subdivisions where the information concerning a given cytogenetic alteration is described and discussed are shown
in the same squares in parentheses. Impossible cases are marked with
"—." Places for conceivable but still unknown cases are vacant.
At first glance it seems astonishing that such important, precise, and
elaborate developmental cytogenetic mechanisms as meiosis and amphimixis exhibit the ability to undergo such far-reaching and manifold
alterations despite their well-stabilized uniformity throughout many
different orders of animals and plants. It is, however, necessary to take
into consideration at least three important points:
(1) During the short period of ontogenesis when these deviations from
the normal course of maturation and fertilization occur, the chromosomes
are not involved in biosynthesis and morphogenesis, and are only passive
carriers of the genetic code. The realization of the hereditary information
begins much later. It is natural that deviations from the usual course of
transference of the hereditary endowment, provided it is finally brought
to the proper place at the necessary time, do not disturb subsequent
processes of biosynthesis and morphogenesis.
B. L. ASTAUROV
different ways of development; the normal process of reproduction is
included.
It is also worth mentioning that each cytogenetic mechanism realized
so far can work with two different physiological modifications of embryonic development, i.e., with or without diapause.
The diapause (or alternatively nondiapause) character of the silkworm
embryo depends on the maternal genetic constitution as well as on
environmental conditions, especially on light and temperature during
the sensitive period comprising the second half of the embryonic development of the mother. Finally, both genetic and environmental factors,
when determining the diapause or nondiapause development, operate
through the neurohormonal regulatory mechanism. Although very important from the standpoint of physiological genetics, this developmental
problem lies beyond the scope of this review (cf. the literature in Astaurov, 1933; Morohoshi, 1957; Tazima, 1964).
A certain degree of ploidy can be obtained and maintained on the
basis of different cytogenetic mechanisms. For instance, the diploid constitution can be achieved in six different ways (Table III, A2, B2, C2,
D2, E2, and F2). Vice versa, the same cytogenetic mechanism could
successfully work with various numbers of chromosome sets, for example,
the ameiotic mode of oocyte maturation in cases of di-, tri-, and tetraploid thermal parthenogenesis (Table III, F2, F3, and F4).
The alterations already obtained and consistent with successful development up to the imago stage are marked in Table III with "-|—f-";
those which can and do arise but are unviable are denoted with " - ) — " ;
references to chapters and subdivisions where the information concerning a given cytogenetic alteration is described and discussed are shown
in the same squares in parentheses. Impossible cases are marked with
"—." Places for conceivable but still unknown cases are vacant.
At first glance it seems astonishing that such important, precise, and
elaborate developmental cytogenetic mechanisms as meiosis and amphimixis exhibit the ability to undergo such far-reaching and manifold
alterations despite their well-stabilized uniformity throughout many
different orders of animals and plants. It is, however, necessary to take
into consideration at least three important points:
(1) During the short period of ontogenesis when these deviations from
the normal course of maturation and fertilization occur, the chromosomes
are not involved in biosynthesis and morphogenesis, and are only passive
carriers of the genetic code. The realization of the hereditary information
begins much later. It is natural that deviations from the usual course of
transference of the hereditary endowment, provided it is finally brought
to the proper place at the necessary time, do not disturb subsequent
processes of biosynthesis and morphogenesis.