290
GERHARD KRAUSE AND KLAUS SANDER
ventro-posteriorly, i.e., from the vegetative or proliferation pole, and
many metameres originate from a segment formation zone (Krause,
1939a). The morphogenetic movements during germ disc formation in
primitive pterygotans are expressions of this primordial egg structure.
Endoplasm contraction in order to 'mould' the ventral germ disc,
streaming of marginal plasm (Section VI B), yolk liquefying function of
vitellophags or simply of mesoderm cells in gastrulation, and cell affinities during all these phases of blastoderm differentiation are synergistic
factors which may act more or less prominently in different egg types.
The formation of embryonic envelopes over germ bands lacking anatrepsis is an indication of the blastoderm's capacity to carry out morphogenetic movements without the aid of the yolk plasmodium. The boatshaped germ discs of Bombyx separated from the yolk system may
gastrulate in vitro and form segments (Krause and Krause, unpublished
data).
B. Anatrepsis
Germ disc formation continuously merges into anatrepsis, i.e., the
shifting of the germ anläge, tail first, into the yolk; both movements
share a common longitudinal component. An attempt to explain anatrepsis was made some time ago by Krause (1938b) but now he considers
it more fruitful to compare that process with the precocious flexure of
the germ band in some myriapod eggs. But whereas in these eggs acron
and telson remain on the surface, in insects the whole germ anläge
disappears within the yolk plasmodium, (e.g., Lepisma), and the amniotic cavity may even lose connection with the serosa (e.g., T achy eines,
Fig. 4a 4 ). In anatrepsis phylogenetically reduced, the front end of the
germ band remains on the ventral side of the egg, whereas the hind end
grows into the yolk on the dorsal side of the egg (e.g., Melasonia, Pig.
5b 4 ); under experimental conditions, however, the germ band of such
eggs may undergo complete anatrepsis (Jura, 1957; W. Schnetter, unpublished data). Conversely, in hemipterans the normally extensive
anatrepsis may be reduced experimentally without harm to further
development (Sander, 1959). Complete anatrepsis is caused by contractions of the yolk plasmodium and not by the growth of the germ band, as
is clearly shown for example in T achy eines where, in diapause eggs, the
transportation of the whole germ disc covered with amnion may be
observed from its beginnings near the posterior pole to its termination
deep inside the yolk behind the CC (Krause, 1938b); the germ anläge, by
contraction of the axial plasm, is pulled even through a narrow 'bottleneck' produced by incomplete constriction behind the CC (Krause,
1958b).
Insight into the dynamics of the yolk plasmodium has been furthered
GERHARD KRAUSE AND KLAUS SANDER
ventro-posteriorly, i.e., from the vegetative or proliferation pole, and
many metameres originate from a segment formation zone (Krause,
1939a). The morphogenetic movements during germ disc formation in
primitive pterygotans are expressions of this primordial egg structure.
Endoplasm contraction in order to 'mould' the ventral germ disc,
streaming of marginal plasm (Section VI B), yolk liquefying function of
vitellophags or simply of mesoderm cells in gastrulation, and cell affinities during all these phases of blastoderm differentiation are synergistic
factors which may act more or less prominently in different egg types.
The formation of embryonic envelopes over germ bands lacking anatrepsis is an indication of the blastoderm's capacity to carry out morphogenetic movements without the aid of the yolk plasmodium. The boatshaped germ discs of Bombyx separated from the yolk system may
gastrulate in vitro and form segments (Krause and Krause, unpublished
data).
B. Anatrepsis
Germ disc formation continuously merges into anatrepsis, i.e., the
shifting of the germ anläge, tail first, into the yolk; both movements
share a common longitudinal component. An attempt to explain anatrepsis was made some time ago by Krause (1938b) but now he considers
it more fruitful to compare that process with the precocious flexure of
the germ band in some myriapod eggs. But whereas in these eggs acron
and telson remain on the surface, in insects the whole germ anläge
disappears within the yolk plasmodium, (e.g., Lepisma), and the amniotic cavity may even lose connection with the serosa (e.g., T achy eines,
Fig. 4a 4 ). In anatrepsis phylogenetically reduced, the front end of the
germ band remains on the ventral side of the egg, whereas the hind end
grows into the yolk on the dorsal side of the egg (e.g., Melasonia, Pig.
5b 4 ); under experimental conditions, however, the germ band of such
eggs may undergo complete anatrepsis (Jura, 1957; W. Schnetter, unpublished data). Conversely, in hemipterans the normally extensive
anatrepsis may be reduced experimentally without harm to further
development (Sander, 1959). Complete anatrepsis is caused by contractions of the yolk plasmodium and not by the growth of the germ band, as
is clearly shown for example in T achy eines where, in diapause eggs, the
transportation of the whole germ disc covered with amnion may be
observed from its beginnings near the posterior pole to its termination
deep inside the yolk behind the CC (Krause, 1938b); the germ anläge, by
contraction of the axial plasm, is pulled even through a narrow 'bottleneck' produced by incomplete constriction behind the CC (Krause,
1958b).
Insight into the dynamics of the yolk plasmodium has been furthered