T H E E A R L Y E M B R Y O G E N E S I S OF I N S E C T S
277
In yolk-rich eggs, which show a well-developed CC (e.g. Tachycines,
Gryllus), it is only the egg shape which limits the irregular expansive
migration of the energids. Following the Vlth cleavage division, the
energids multiply below the egg surface which forces them into the
spheroid arrangement until, during the I X t h cleavage division, they
leave the marginal plasm for blastoderm formation. In plasma-rich eggs
with stronger differences between ecto- and endoplasm, the energids
behave similarly, but the spheroid formed after the Vlth cleavage is
situated more internally, and during the I X t h division the energids are
still approaching the periplasm. If the controlling centre is situated close
to one egg pole, the longitudinal component of movement for spheroid
formation will be supplemented by an additional influence.
B. Vitellophags
Typically, the vitellophags segregate after the Vlth and V l l t h cleavage divisions. They soon become marked by the size of nucleus, size of
plasma island, mitosis, or endomitosis. Primary vitellophags are left behind in typical places and at a typical time in the axial endoplasm (in
the CC in Tachycines and others, in the DC in Apis). They do not take
part in the formation of the spheroid of energids. Secondary vitellophags
originate by non-tangential mitoses of the spheroid energids and remain
in the marginal endoplasm, or they enter this plasm after leaving the
blastoderm. Probably those energids which could not collect sufficient
ectoplasm or did not gain contact with the plasmalemma turn into
secondary vitellophags. They may segregate successively, for example
beginning dorsally in the CC region (Tachycines, Fig. 4a 2 ). In Calliphora
they segregate first from the posterior pole, and later from the anterior
pole (Fig. 8c, d); in ligated eggs, supernumerary secondary vitellophags
enter the yolk in front of and behind the ligature (Nitschmann, 1959;
Fig. 8i). Tertiary vitellophags in Apis break down the yolk in those
places where formation of germ layers proceeds (Müller, 1957; YC in
Fig. 3e). Both processes start from the DC. The yolk plasmodium and
vitellophags take part in anatrepsis and yolk cleavage. The yolk plasmodium including the vitellophags may be reduced (e.g. Apiomorpha;
Büchner, 1957) or even absent as in polyembryonic Hymenoptera.
C. Cleavage in the Future Thorax Region
In some insects the pronuclei and first cleavage nuclei are situated in
the presumptive thoracic region instead of in the procephalic region,
as is normally the case (e.g. Culex; Idris, 1960a; Oncopeltus; Meyer,
unpublished). Three hypothetical explanations for this characteristic
have been put forward by Krause (1961). The assumption of a backward
movement of the oocyte nucleus, similar to the migration of cleavage
K
A.M. 2
277
In yolk-rich eggs, which show a well-developed CC (e.g. Tachycines,
Gryllus), it is only the egg shape which limits the irregular expansive
migration of the energids. Following the Vlth cleavage division, the
energids multiply below the egg surface which forces them into the
spheroid arrangement until, during the I X t h cleavage division, they
leave the marginal plasm for blastoderm formation. In plasma-rich eggs
with stronger differences between ecto- and endoplasm, the energids
behave similarly, but the spheroid formed after the Vlth cleavage is
situated more internally, and during the I X t h division the energids are
still approaching the periplasm. If the controlling centre is situated close
to one egg pole, the longitudinal component of movement for spheroid
formation will be supplemented by an additional influence.
B. Vitellophags
Typically, the vitellophags segregate after the Vlth and V l l t h cleavage divisions. They soon become marked by the size of nucleus, size of
plasma island, mitosis, or endomitosis. Primary vitellophags are left behind in typical places and at a typical time in the axial endoplasm (in
the CC in Tachycines and others, in the DC in Apis). They do not take
part in the formation of the spheroid of energids. Secondary vitellophags
originate by non-tangential mitoses of the spheroid energids and remain
in the marginal endoplasm, or they enter this plasm after leaving the
blastoderm. Probably those energids which could not collect sufficient
ectoplasm or did not gain contact with the plasmalemma turn into
secondary vitellophags. They may segregate successively, for example
beginning dorsally in the CC region (Tachycines, Fig. 4a 2 ). In Calliphora
they segregate first from the posterior pole, and later from the anterior
pole (Fig. 8c, d); in ligated eggs, supernumerary secondary vitellophags
enter the yolk in front of and behind the ligature (Nitschmann, 1959;
Fig. 8i). Tertiary vitellophags in Apis break down the yolk in those
places where formation of germ layers proceeds (Müller, 1957; YC in
Fig. 3e). Both processes start from the DC. The yolk plasmodium and
vitellophags take part in anatrepsis and yolk cleavage. The yolk plasmodium including the vitellophags may be reduced (e.g. Apiomorpha;
Büchner, 1957) or even absent as in polyembryonic Hymenoptera.
C. Cleavage in the Future Thorax Region
In some insects the pronuclei and first cleavage nuclei are situated in
the presumptive thoracic region instead of in the procephalic region,
as is normally the case (e.g. Culex; Idris, 1960a; Oncopeltus; Meyer,
unpublished). Three hypothetical explanations for this characteristic
have been put forward by Krause (1961). The assumption of a backward
movement of the oocyte nucleus, similar to the migration of cleavage
K
A.M. 2