CYTOGENETIC MECHANISMS IN SILKWORMS
217
of parthenogenetic cleavage occur: (1) in the absence of the centrosome
which is normally introduced by the sperm nucleus, and (2) in abnormally "strong" nucleocytoplasmic ratio of the first haploid blastomere
and of its parthenogenetic derivatives.
However, it seems evident that the presence of the sperm centrosome
is by no means indispensable for a normal parthenogenetic cleavage and
for subsequent successful development. This can be inferred from the
fact of complete parthenogenetic development, which as we have seen
and shall see below, is quite possible, provided the haploidy of cleavage
nuclei is avoided from the very beginning of segmentation either by
restoration of diploidy (in cases of meiotic and automictic parthenogenesis) or by the preservation of the somatic chromosome number (in cases
of ameiotic, diploid, and polyploid thermal parthenogenesis).
As to the importance of a "strong" nucleocytoplasmic ratio, it is not
so easy to decide which of many abnormal characteristics inherent in
haploid-reproducing blastomeres may be taken as the primary cause of
disturbances in segmentation. Is it the abnormal nucleocytoplasmic
ratio, abnormal metabolic interrelations of haploid nucleus with its
cytoplasmic surrounding, or the abnormal interaction of the haploid
blastomere as a whole and the yolk of the ovum? For the time being,
it seems more cautious to leave these questions open and to restrict our
conclusion to the statement that the abnormality of development is
somehow connected with the haploidy. It is also clear that in spite of
the previously stressed morphogenetic significance of the preembryonic
organization of the future embryo (Section I,A) this predetermination
alone is unable to accomplish further morphogenesis without proper
cooperation with an adequate genetic system, the orthodiploid or (Section
V,C) orthopolyploid chromosome constitution. Therefore, in order to
be effective the method of complete artificial parthenogenesis should
meet at least two indispensable requirements: (1) to induce the initiation of a generalized activation reaction in an unfertilized egg, and (2)
to ensure the preservation of the diploid (or polyploid) chromosome
constitution of the unreduced oocyte or the restoration of diploidy (or
polyploidy). Thermoactivation of unfertilized eggs by means of a heat
shock as developed by Astaurov (1936a, 1940) remains up to now the
sole experimental procedure which can simultaneously accomplish both
above-mentioned tasks and induce complete parthenogenetic development.
C. Artificial Parthenogenesis by Heat-Shock Treatment
1. Procedure and Physiological Aspects of Thermoactivation
High temperature was shown to be a powerful parthenogenetic agent.
The effect of hot water has already been noted by Tichomiroff in his
217
of parthenogenetic cleavage occur: (1) in the absence of the centrosome
which is normally introduced by the sperm nucleus, and (2) in abnormally "strong" nucleocytoplasmic ratio of the first haploid blastomere
and of its parthenogenetic derivatives.
However, it seems evident that the presence of the sperm centrosome
is by no means indispensable for a normal parthenogenetic cleavage and
for subsequent successful development. This can be inferred from the
fact of complete parthenogenetic development, which as we have seen
and shall see below, is quite possible, provided the haploidy of cleavage
nuclei is avoided from the very beginning of segmentation either by
restoration of diploidy (in cases of meiotic and automictic parthenogenesis) or by the preservation of the somatic chromosome number (in cases
of ameiotic, diploid, and polyploid thermal parthenogenesis).
As to the importance of a "strong" nucleocytoplasmic ratio, it is not
so easy to decide which of many abnormal characteristics inherent in
haploid-reproducing blastomeres may be taken as the primary cause of
disturbances in segmentation. Is it the abnormal nucleocytoplasmic
ratio, abnormal metabolic interrelations of haploid nucleus with its
cytoplasmic surrounding, or the abnormal interaction of the haploid
blastomere as a whole and the yolk of the ovum? For the time being,
it seems more cautious to leave these questions open and to restrict our
conclusion to the statement that the abnormality of development is
somehow connected with the haploidy. It is also clear that in spite of
the previously stressed morphogenetic significance of the preembryonic
organization of the future embryo (Section I,A) this predetermination
alone is unable to accomplish further morphogenesis without proper
cooperation with an adequate genetic system, the orthodiploid or (Section
V,C) orthopolyploid chromosome constitution. Therefore, in order to
be effective the method of complete artificial parthenogenesis should
meet at least two indispensable requirements: (1) to induce the initiation of a generalized activation reaction in an unfertilized egg, and (2)
to ensure the preservation of the diploid (or polyploid) chromosome
constitution of the unreduced oocyte or the restoration of diploidy (or
polyploidy). Thermoactivation of unfertilized eggs by means of a heat
shock as developed by Astaurov (1936a, 1940) remains up to now the
sole experimental procedure which can simultaneously accomplish both
above-mentioned tasks and induce complete parthenogenetic development.
C. Artificial Parthenogenesis by Heat-Shock Treatment
1. Procedure and Physiological Aspects of Thermoactivation
High temperature was shown to be a powerful parthenogenetic agent.
The effect of hot water has already been noted by Tichomiroff in his