274
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
35%), the activation centre of Apis (Fig. 3) may already exert its influence during oogenesis. Identical processes are seen to take place in
eggs of different species at different times and on different organizational
levels.
IV. Superficial Cleavage
During cleavage of insect eggs, a number of processes may display
more or less strongly alternative tendencies. Cleavage mitoses may
occur synchronously or heterochronously, the diameter of nuclei may
increase or decrease, plasmic islands may be formed by absorption of
endoplasm or by assimilation of yolk, the cleavage nuclei may migrate
independently of each other or as a group, cells may contain yolk or be
free of it, and the germ anläge may be preformed in the ooplasm or
appear epigenetically by differentiation of primarily identical energids
and blastoderm cells. The time correlation of these processes may be
different even in closely related species (Seidler, 1940; Buchner, 1957).
However, the few cells resulting from mosaic cleavage in the coccid
Apiomorpha urnalis (Büchner, 1957) undergo the same processes of
somatic differentiation as do the numerous energids in other insect eggs;
changes of ooplasm distribution during synchronous cleavage in every
case pave the way for differentiation of nuclei and cells. In a typical
case, represented by the megalopteran Sialis (Du Bois, 1938) and, in a
similar way, by the hemipteran Notonecta (Fig. 4c), several phases of
plasmodial development may be discerned. These are characterized as
follows:
1. Synchronous mitoses I—III: dichotomous distribution of daughter
nuclei around the site of the zygote nucleus; formation of plasma islands
from endoplasm. This phase is controlled presumably by the cleavage
centre.
2. Synchronous mitoses IV-VI: collective backward movement either
of all energids (Fig. 4b!), or of the hindmost energids arranged in a
transverse plane (Fig. 4c 1? Fig. 3b); oosome fragmentation; formation or
reinforcement of periplasm. This phase is controlled presumably by
longitudinal differences or from the egg poles, perhaps including the
activation centre.
3. Synchronous mitoses V I I - I X : energids approach periplasm in
spheroid arrangement; primary vitellophags lag behind; secondary
periplasm migrates towards preblastoderm; secondary vitellophags move
back into the yolk plasmodium. This phase presumably is controlled by
differences between ecto- and endoplasm.
4. Synchronous or heterochronous mitoses in blastoderm and vitellophags ; formation of pole cells and ventral germ anläge.
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
35%), the activation centre of Apis (Fig. 3) may already exert its influence during oogenesis. Identical processes are seen to take place in
eggs of different species at different times and on different organizational
levels.
IV. Superficial Cleavage
During cleavage of insect eggs, a number of processes may display
more or less strongly alternative tendencies. Cleavage mitoses may
occur synchronously or heterochronously, the diameter of nuclei may
increase or decrease, plasmic islands may be formed by absorption of
endoplasm or by assimilation of yolk, the cleavage nuclei may migrate
independently of each other or as a group, cells may contain yolk or be
free of it, and the germ anläge may be preformed in the ooplasm or
appear epigenetically by differentiation of primarily identical energids
and blastoderm cells. The time correlation of these processes may be
different even in closely related species (Seidler, 1940; Buchner, 1957).
However, the few cells resulting from mosaic cleavage in the coccid
Apiomorpha urnalis (Büchner, 1957) undergo the same processes of
somatic differentiation as do the numerous energids in other insect eggs;
changes of ooplasm distribution during synchronous cleavage in every
case pave the way for differentiation of nuclei and cells. In a typical
case, represented by the megalopteran Sialis (Du Bois, 1938) and, in a
similar way, by the hemipteran Notonecta (Fig. 4c), several phases of
plasmodial development may be discerned. These are characterized as
follows:
1. Synchronous mitoses I—III: dichotomous distribution of daughter
nuclei around the site of the zygote nucleus; formation of plasma islands
from endoplasm. This phase is controlled presumably by the cleavage
centre.
2. Synchronous mitoses IV-VI: collective backward movement either
of all energids (Fig. 4b!), or of the hindmost energids arranged in a
transverse plane (Fig. 4c 1? Fig. 3b); oosome fragmentation; formation or
reinforcement of periplasm. This phase is controlled presumably by
longitudinal differences or from the egg poles, perhaps including the
activation centre.
3. Synchronous mitoses V I I - I X : energids approach periplasm in
spheroid arrangement; primary vitellophags lag behind; secondary
periplasm migrates towards preblastoderm; secondary vitellophags move
back into the yolk plasmodium. This phase presumably is controlled by
differences between ecto- and endoplasm.
4. Synchronous or heterochronous mitoses in blastoderm and vitellophags ; formation of pole cells and ventral germ anläge.