T H E E A R L Y E M B R Y O G E N E S I S O F I N S E C T S
297
(f 3 ) or behind it during the late blastoderm; in this case, a complete
embryo with normal polarity may form in the anterior isolate (e 3 ).
Obviously, in these experiments the character of a germ band segment
is influenced by its distance in either direction from the posterior pole
material (arrows between d and e). This material therefore must build
up a gradient of a posterior factor (p-factor) around itself, a conclusion
supported by a number of observations not mentioned here. Considerations based on less striking results have led to a similar conclusion
concerning the anterior factor (a-factor) (Sander, 1960). The relative
strength of each factor in an egg region is thought to decide the fate of
the blastoderm there; this view also accounts for the antagonistic influence of the factors upon the blastodermal locus of germ band segments, as illustrated by the prothorax in Fig. 9b 2 where the p-factor was
insufficient, in d x where the situation was normal, and in c 2 where the
a-factor was insufficient.
The experimentally demonstrated a- and p-factors may be connected
with CC and AC, but as yet they cannot be identified with definite
constituents of the egg. The cleavage energids are unlikely to carry the
anterior factor backwards. The spread of p-factor from the posterior
pole region starts at, or perhaps even before, oviposition; therefore it is
not known, whether or how the AC has to be activated beforehand.
Division of the ooplasmodium by releasing the loop immediately after
egg constriction leads to abnormal development and finally decay,
although the shape and internal pressure of the egg are restored after the
operation. Thus, continuity of the ooplasm appears to be a precondition
for the a-p-reaction system to function.
Longitudinal mirror-type reduplication of terminal germ band parts
(as in Euscelis, Fig. 9d 2 ), and even of head lobes has been obtained by
Yajima (1960) who recombined egg constituents of Chironomus by
means of centrifugation; Price (1958) found posterior double monsters
of the same type to occur frequently in a species of Wyeomyia. Yajima's
interpretation of his findings is in some respects in good agreement with
the conclusions derived from the results obtained in Euscelis, and we
may hope that this common basis will help to find a workable explanation for epigenesis of metameric organization in other insect eggs too.
C. Initial Concentration of Potentialities
In Apis eggs ligatured during early stages, material situated immediately in front of the constriction possibly is used for body segments
or tagmata normally situated more anteriorly, whereas material close
behind the ligature may form embryonic parts normally developing in
more posterior regions, as in Calliphora. Schnetter (1934b) therefore dis-
297
(f 3 ) or behind it during the late blastoderm; in this case, a complete
embryo with normal polarity may form in the anterior isolate (e 3 ).
Obviously, in these experiments the character of a germ band segment
is influenced by its distance in either direction from the posterior pole
material (arrows between d and e). This material therefore must build
up a gradient of a posterior factor (p-factor) around itself, a conclusion
supported by a number of observations not mentioned here. Considerations based on less striking results have led to a similar conclusion
concerning the anterior factor (a-factor) (Sander, 1960). The relative
strength of each factor in an egg region is thought to decide the fate of
the blastoderm there; this view also accounts for the antagonistic influence of the factors upon the blastodermal locus of germ band segments, as illustrated by the prothorax in Fig. 9b 2 where the p-factor was
insufficient, in d x where the situation was normal, and in c 2 where the
a-factor was insufficient.
The experimentally demonstrated a- and p-factors may be connected
with CC and AC, but as yet they cannot be identified with definite
constituents of the egg. The cleavage energids are unlikely to carry the
anterior factor backwards. The spread of p-factor from the posterior
pole region starts at, or perhaps even before, oviposition; therefore it is
not known, whether or how the AC has to be activated beforehand.
Division of the ooplasmodium by releasing the loop immediately after
egg constriction leads to abnormal development and finally decay,
although the shape and internal pressure of the egg are restored after the
operation. Thus, continuity of the ooplasm appears to be a precondition
for the a-p-reaction system to function.
Longitudinal mirror-type reduplication of terminal germ band parts
(as in Euscelis, Fig. 9d 2 ), and even of head lobes has been obtained by
Yajima (1960) who recombined egg constituents of Chironomus by
means of centrifugation; Price (1958) found posterior double monsters
of the same type to occur frequently in a species of Wyeomyia. Yajima's
interpretation of his findings is in some respects in good agreement with
the conclusions derived from the results obtained in Euscelis, and we
may hope that this common basis will help to find a workable explanation for epigenesis of metameric organization in other insect eggs too.
C. Initial Concentration of Potentialities
In Apis eggs ligatured during early stages, material situated immediately in front of the constriction possibly is used for body segments
or tagmata normally situated more anteriorly, whereas material close
behind the ligature may form embryonic parts normally developing in
more posterior regions, as in Calliphora. Schnetter (1934b) therefore dis-