298
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
cerned 'Keimbezirke' (areas of the tagmata as revealed by mosaic development following operation in late blastoderm stages, Fig. 3c) and
'Potenzbereiche' (regions capable of forming those tagmata after operation in early blastoderm stages). The posterior borders of the Potenz -
bereiche appear closer to the prothoracic DC than those of the definitive
areas. The DC was therefore thought to be a place of concentration of
organogenetic potentialities, notwithstanding the fact that the location
of the anterior borders, if such exist, of the Potenzbereiche as yet had to
be established. Using the conception of morphogenetic fields for the
different tagmata, one might state that these overlap in the DC during
the early blastoderm, whereas later they have become segregated into
the proper sequence. This segregation, or the ability of blastoderm cells
to concentrate (a precondition for further differentiation), may depend
upon the spreading effects (Fig. 3, curve V) of an abdominal control
centre (Sauer, 1954). In addition to these conditions, information for
matameric organization could be stored in the egg in a way not congruent with the fate map worked out for later blastoderm stages; such
information might originate form the nurse cells which, during 'proliferative' oocyte growth (see Section I I I D), may produce successively
different RNA combinations.
Initial concentration of potentialities has been put forward as an
explanation for results obtained after U.V.-irradiation by Brauer (1949)
and, in a more elaborate way, by Seidel (1961). In Platycnemis, irradiation of a strip of lateral ectoplasm during the early 512-nuclei stage
causes defects in gnathic and thoracic segments (Fig. 6d). Defects of the
same type are observed if regions more distant from the DC have been
hit during the early blastoderm, 5 hours later (Fig. 6d). Potentialities
originally concentrated in the DC region are thought therefore to have
spread out before blastoderm formation was complete. Seidel's operations unfortunately not only destroyed peripheral plasm structures
but, at least during the late stage, also nuclei. Since nuclei have been
damaged, the results might not indicate an actual spread of prerequisites
in the cortex, but rather the incompetence of the irradiated nuclei to
react to information which they encounter on approaching the DC
region by blastoderm concentration or mitotic pattern for germ disc
formation. On the other hand, the periplasmic pattern might not consist
of qualitatively different information for each metamere, but primarily
of only two different influences spreading in opposite directions and first
passing each other in f he DC. If, in agreement with Sander's tentative
explanation (Section VII B), one supposes different body regions to be
determined according to the relative strength of the anterior and posterior influences, Seidel's results could be explained by assuming that
both influences at first are graded steeply, but flatten out somewhat
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
cerned 'Keimbezirke' (areas of the tagmata as revealed by mosaic development following operation in late blastoderm stages, Fig. 3c) and
'Potenzbereiche' (regions capable of forming those tagmata after operation in early blastoderm stages). The posterior borders of the Potenz -
bereiche appear closer to the prothoracic DC than those of the definitive
areas. The DC was therefore thought to be a place of concentration of
organogenetic potentialities, notwithstanding the fact that the location
of the anterior borders, if such exist, of the Potenzbereiche as yet had to
be established. Using the conception of morphogenetic fields for the
different tagmata, one might state that these overlap in the DC during
the early blastoderm, whereas later they have become segregated into
the proper sequence. This segregation, or the ability of blastoderm cells
to concentrate (a precondition for further differentiation), may depend
upon the spreading effects (Fig. 3, curve V) of an abdominal control
centre (Sauer, 1954). In addition to these conditions, information for
matameric organization could be stored in the egg in a way not congruent with the fate map worked out for later blastoderm stages; such
information might originate form the nurse cells which, during 'proliferative' oocyte growth (see Section I I I D), may produce successively
different RNA combinations.
Initial concentration of potentialities has been put forward as an
explanation for results obtained after U.V.-irradiation by Brauer (1949)
and, in a more elaborate way, by Seidel (1961). In Platycnemis, irradiation of a strip of lateral ectoplasm during the early 512-nuclei stage
causes defects in gnathic and thoracic segments (Fig. 6d). Defects of the
same type are observed if regions more distant from the DC have been
hit during the early blastoderm, 5 hours later (Fig. 6d). Potentialities
originally concentrated in the DC region are thought therefore to have
spread out before blastoderm formation was complete. Seidel's operations unfortunately not only destroyed peripheral plasm structures
but, at least during the late stage, also nuclei. Since nuclei have been
damaged, the results might not indicate an actual spread of prerequisites
in the cortex, but rather the incompetence of the irradiated nuclei to
react to information which they encounter on approaching the DC
region by blastoderm concentration or mitotic pattern for germ disc
formation. On the other hand, the periplasmic pattern might not consist
of qualitatively different information for each metamere, but primarily
of only two different influences spreading in opposite directions and first
passing each other in f he DC. If, in agreement with Sander's tentative
explanation (Section VII B), one supposes different body regions to be
determined according to the relative strength of the anterior and posterior influences, Seidel's results could be explained by assuming that
both influences at first are graded steeply, but flatten out somewhat