296
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
anterior isolates can do so only if the ligature is situated very close to the
posterior pole. Smaller isolates, anterior (b^-bg) as well as posterior
(c 2 -c 3 ), with increasing age at the time of operation, gain increased
differentiating capacity as revealed by an increase in the number of
segments formed. Consequently, ligature within the posterior two-thirds
of the egg results in loss of many germ band segments if carried out
during cleavage, whereas in eggs constricted shortly before germ anläge .
formation only one or two segments are lacking (d 3 ).
Since 'decrease of sensibility' as an explanation is excluded by other
experiments (d-f), these results are taken to indicate a spread of anterior
and posterior prerequisites for the determination of segment loci in the
blastoderm. As indicated between Figs. 9b and 9c, during maturation
(a x ) the posterior factor ('ρ') is limited to the posterior pole region, and
the anterior factor ('a') is located somewhere in front of the 60% level.
Until late cleavage (a 2 ) the factors have begun to spread towards each
other (arrows); when the head lobes appear in the blastoderm (a 3 ), the
spread has practically come to an end, whereas the segments, possibly
starting from a DC, begin to segregate in the metameric pattern predisposed by the anterior and posterior prerequisites. Partial constriction
of the egg, although apparently slowing down the progress of the anterior factor, does not prevent its spread altogether; the germ anläge
appears in normal position (b^.
The influence of the posterior pole material upon metameric organization, which was inferred from defective development in anterior isolates
(b l5 b 2 ) and in eggs teratologically lacking this posterior material (ί χ ),
can positively be demonstrated by translocation of this material (p in
circle) to the posterior end of an anterior isolate comprising about twothirds of the egg. If carried out before the late blastoderm stages, this
operation very frequently will result in formation of a complete embryo
in an egg part which otherwise at best would have produced the head
lobes (e l5 e 2 ). If the posterior isolate of the same egg contains nuclei, it
may form additional germ band parts (e 2 ); this duplicitas longitudinalis
is irrefutable proof of the fact that in Euscelis no metameric organization
pattern is predetermined, as might be suggested by the partial embryos
in isolates.
Upon the type of influence exerted by the posterior pole material,
some light is shed by another combination of ooplasmic materials. If the
posterior pole material is put into the front end of a posterior isolate of
certain dimensions, the sequence of germ band segments formed there
will be partially reversed (b 2 ; reduplication with transversal plane of
symmetry) or completely reversed. The same result in a smaller isolate
(e 3 ; complete reversal) may be obtained if the posterior pole material is
pushed forward during cleavage (f 2 ) and the egg is ligatured in front of
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
anterior isolates can do so only if the ligature is situated very close to the
posterior pole. Smaller isolates, anterior (b^-bg) as well as posterior
(c 2 -c 3 ), with increasing age at the time of operation, gain increased
differentiating capacity as revealed by an increase in the number of
segments formed. Consequently, ligature within the posterior two-thirds
of the egg results in loss of many germ band segments if carried out
during cleavage, whereas in eggs constricted shortly before germ anläge .
formation only one or two segments are lacking (d 3 ).
Since 'decrease of sensibility' as an explanation is excluded by other
experiments (d-f), these results are taken to indicate a spread of anterior
and posterior prerequisites for the determination of segment loci in the
blastoderm. As indicated between Figs. 9b and 9c, during maturation
(a x ) the posterior factor ('ρ') is limited to the posterior pole region, and
the anterior factor ('a') is located somewhere in front of the 60% level.
Until late cleavage (a 2 ) the factors have begun to spread towards each
other (arrows); when the head lobes appear in the blastoderm (a 3 ), the
spread has practically come to an end, whereas the segments, possibly
starting from a DC, begin to segregate in the metameric pattern predisposed by the anterior and posterior prerequisites. Partial constriction
of the egg, although apparently slowing down the progress of the anterior factor, does not prevent its spread altogether; the germ anläge
appears in normal position (b^.
The influence of the posterior pole material upon metameric organization, which was inferred from defective development in anterior isolates
(b l5 b 2 ) and in eggs teratologically lacking this posterior material (ί χ ),
can positively be demonstrated by translocation of this material (p in
circle) to the posterior end of an anterior isolate comprising about twothirds of the egg. If carried out before the late blastoderm stages, this
operation very frequently will result in formation of a complete embryo
in an egg part which otherwise at best would have produced the head
lobes (e l5 e 2 ). If the posterior isolate of the same egg contains nuclei, it
may form additional germ band parts (e 2 ); this duplicitas longitudinalis
is irrefutable proof of the fact that in Euscelis no metameric organization
pattern is predetermined, as might be suggested by the partial embryos
in isolates.
Upon the type of influence exerted by the posterior pole material,
some light is shed by another combination of ooplasmic materials. If the
posterior pole material is put into the front end of a posterior isolate of
certain dimensions, the sequence of germ band segments formed there
will be partially reversed (b 2 ; reduplication with transversal plane of
symmetry) or completely reversed. The same result in a smaller isolate
(e 3 ; complete reversal) may be obtained if the posterior pole material is
pushed forward during cleavage (f 2 ) and the egg is ligatured in front of