C Y T O G E N E T I C M E C H A N I S M S I N S I L K W O R M S
205
cytoplasmic island associated with the cortex. There is a physiological
polyspermy in Bombyx; small bunches of spermatozoa are "injected"
through the micropyle into the interior of the egg by means of a very
elaborate biological mechanism (Strunnikov, 1959). According to the
old data of Kawaguchi (1926), the observed number of spermatozoa
inside the ooplasm fluctuates between 1 and 11, the monospermic inseminations comprising only 13.8%, while 85% of fertilized ova contain
2-5 spermatozoa. Immediately after the penetration of spermatozoa and
practically simultaneously with the deposition of the egg, the maturation
blockage is broken and the process of meiotic divisions recommences.
Division I is completed in 1 hr; depending on the genetic constitution
of the race and external temperature, metaphase II is formed between 1
hr and 15 min and 1 hr and 45 min. Three polar nuclei are thrown
out and finally disintegrate on the egg surface while the fourth innermost
nucleus becomes the female pronucleus and migrates to the egg center.
At 2.5-3.5 hr, the copulation of pronuclei takes place. The syncarion is
located in the depths of the cytoplasm in the longitudinal axis of the egg
at a distance of one-fifth to one-fourth of its total length from the micropylar pole.
Cleavage begins at 3.5-4.5 hr. The formation of several male pronuclei
proceeds completely synchronously but only one male pronucleus, that
nearest to the female pronucleus, participates in the fusion process. When
synchronous cleavage begins, supernumerary haploid male pronuclei
usually form metaphase plates and try to undergo mitotic division. However, haploid division is never terminated and finally all supernumerary
pronuclei disintegrate. With respect to division of such haploid spermatozoa, a very peculiar process of cytoplasmic and centrosome division
without simultaneous reduplication of the chromosomes has been described (Bataillon and Tchou-Su, 1933; Tchou-Su and Chang-Ko, 1949,
1955). According to their observations, cytoplasmic islets containing
centrosomes detach from the blocked haploid spindles and possess the
ability to continue further autonomous reproduction.
The first maturation division in the silkworm egg, as in other Lepidoptera and some other insect orders, has a distinctive character: formation of the so-called elimination chromatin. This substance, actually
Feulgen-negative and having a strong affinity for basic dyes, is shed
from the chromosomes when they begin their anaphase movement
towards the poles, and remains lying in the place previously occupied by
the metaphase plate. No such elimination plate is formed during the
second maturation division.
According to Frolova (1940a), just before the recommencement of the
first maturation division, the tetrads are situated in the blocked metaphase plate in such a position that the reductional split between con-
205
cytoplasmic island associated with the cortex. There is a physiological
polyspermy in Bombyx; small bunches of spermatozoa are "injected"
through the micropyle into the interior of the egg by means of a very
elaborate biological mechanism (Strunnikov, 1959). According to the
old data of Kawaguchi (1926), the observed number of spermatozoa
inside the ooplasm fluctuates between 1 and 11, the monospermic inseminations comprising only 13.8%, while 85% of fertilized ova contain
2-5 spermatozoa. Immediately after the penetration of spermatozoa and
practically simultaneously with the deposition of the egg, the maturation
blockage is broken and the process of meiotic divisions recommences.
Division I is completed in 1 hr; depending on the genetic constitution
of the race and external temperature, metaphase II is formed between 1
hr and 15 min and 1 hr and 45 min. Three polar nuclei are thrown
out and finally disintegrate on the egg surface while the fourth innermost
nucleus becomes the female pronucleus and migrates to the egg center.
At 2.5-3.5 hr, the copulation of pronuclei takes place. The syncarion is
located in the depths of the cytoplasm in the longitudinal axis of the egg
at a distance of one-fifth to one-fourth of its total length from the micropylar pole.
Cleavage begins at 3.5-4.5 hr. The formation of several male pronuclei
proceeds completely synchronously but only one male pronucleus, that
nearest to the female pronucleus, participates in the fusion process. When
synchronous cleavage begins, supernumerary haploid male pronuclei
usually form metaphase plates and try to undergo mitotic division. However, haploid division is never terminated and finally all supernumerary
pronuclei disintegrate. With respect to division of such haploid spermatozoa, a very peculiar process of cytoplasmic and centrosome division
without simultaneous reduplication of the chromosomes has been described (Bataillon and Tchou-Su, 1933; Tchou-Su and Chang-Ko, 1949,
1955). According to their observations, cytoplasmic islets containing
centrosomes detach from the blocked haploid spindles and possess the
ability to continue further autonomous reproduction.
The first maturation division in the silkworm egg, as in other Lepidoptera and some other insect orders, has a distinctive character: formation of the so-called elimination chromatin. This substance, actually
Feulgen-negative and having a strong affinity for basic dyes, is shed
from the chromosomes when they begin their anaphase movement
towards the poles, and remains lying in the place previously occupied by
the metaphase plate. No such elimination plate is formed during the
second maturation division.
According to Frolova (1940a), just before the recommencement of the
first maturation division, the tetrads are situated in the blocked metaphase plate in such a position that the reductional split between con-