280
G E R H A R D K R A U S E AND K L A U S S A N D E R
insect egg cell or ooplasmodium, structural elements such as nuclei or
plasma islands or cell organelles or macromolecules have to come together to enable epigenesis to take place. Seidel was the first to demonstrate experimentally a reaction sequence in the insect egg and to point
out the general importance for epigenesis of a fixed sequence of movements and activations or inductions. The proof of an activation by
elimination of one of the partners of the primary reaction at the proper
time or by translocation of a reaction partner to an atypical place
depends upon a chain of subsequent reactions which are subject to the
law of 'all or nothing'. Only exceptionally favourable circumstances
will allow the isolation or translocation of a well-localized partner, thus
permitting elucidation of the primary reaction with its neighbours; in
one egg type there may be immediate reaction with a competent partner
in loco which in another egg type is distributed throughout the egg. Thus
it is very difficult to analyse the whole cascade of reactions and to
characterize and homologize factor regions in the insect egg.
A. A Type of Reaction Sequence
Seidel's well-known investigations (1926-1936) on the egg of Platycnemis (Odonata) are briefly summed up in Fig. 6. One of 64 or 128
energids has to enter the ooplasm at or behind the 10% level and there
to start a reaction which is followed by a change in the yolk endoplasm
system spreading anteriorly. If the activation of the preformed 'Bildungszentrum' (=AC) is prevented by keeping away the energids by
means of partial constriction of the egg, neither a complete nor a partial
germ anläge is formed. The same is effected if the AC be removed by
ligation or by cautery of the posterior pole region. Until cleavage VII,
the indispensable region near the posterior pole does not increase. Thereafter, embryonic differentiation can be suppressed only by removing an
increasingly bigger posterior part of the egg (curve II). This means that
a formative factor has spread in an anterior direction. This process may
take place even if no structural, but only a humoral, connection exists
between the activated posterior pole region and the rest of the egg, as is
shown by embryonic development of eggs constricted almost completely
after a cleavage energid has entered the AC. Therefore, the activated AC
must give off a substance previously stored there, or the product of a
reaction taking place in the centre must move anteriorly, or a chain of
reactions must start in this direction. After the 256-nuclei stage, the
entire ooplasm system shows a change in consistency and is less sensitive
to ligature than before. Curve I I I indicates the spread of increasing
translucency of the yolk plasm during early blastoderm stages. Formation of the blastoderm and later on of yolk cells does not depend
upon influences from the AC. The yolk plasmodium, however, gains its
G E R H A R D K R A U S E AND K L A U S S A N D E R
insect egg cell or ooplasmodium, structural elements such as nuclei or
plasma islands or cell organelles or macromolecules have to come together to enable epigenesis to take place. Seidel was the first to demonstrate experimentally a reaction sequence in the insect egg and to point
out the general importance for epigenesis of a fixed sequence of movements and activations or inductions. The proof of an activation by
elimination of one of the partners of the primary reaction at the proper
time or by translocation of a reaction partner to an atypical place
depends upon a chain of subsequent reactions which are subject to the
law of 'all or nothing'. Only exceptionally favourable circumstances
will allow the isolation or translocation of a well-localized partner, thus
permitting elucidation of the primary reaction with its neighbours; in
one egg type there may be immediate reaction with a competent partner
in loco which in another egg type is distributed throughout the egg. Thus
it is very difficult to analyse the whole cascade of reactions and to
characterize and homologize factor regions in the insect egg.
A. A Type of Reaction Sequence
Seidel's well-known investigations (1926-1936) on the egg of Platycnemis (Odonata) are briefly summed up in Fig. 6. One of 64 or 128
energids has to enter the ooplasm at or behind the 10% level and there
to start a reaction which is followed by a change in the yolk endoplasm
system spreading anteriorly. If the activation of the preformed 'Bildungszentrum' (=AC) is prevented by keeping away the energids by
means of partial constriction of the egg, neither a complete nor a partial
germ anläge is formed. The same is effected if the AC be removed by
ligation or by cautery of the posterior pole region. Until cleavage VII,
the indispensable region near the posterior pole does not increase. Thereafter, embryonic differentiation can be suppressed only by removing an
increasingly bigger posterior part of the egg (curve II). This means that
a formative factor has spread in an anterior direction. This process may
take place even if no structural, but only a humoral, connection exists
between the activated posterior pole region and the rest of the egg, as is
shown by embryonic development of eggs constricted almost completely
after a cleavage energid has entered the AC. Therefore, the activated AC
must give off a substance previously stored there, or the product of a
reaction taking place in the centre must move anteriorly, or a chain of
reactions must start in this direction. After the 256-nuclei stage, the
entire ooplasm system shows a change in consistency and is less sensitive
to ligature than before. Curve I I I indicates the spread of increasing
translucency of the yolk plasm during early blastoderm stages. Formation of the blastoderm and later on of yolk cells does not depend
upon influences from the AC. The yolk plasmodium, however, gains its