T H E E A R L Y E M B R Y O G E N E S I S OF I N S E C T S
293
differentiation in the ectoplasm should be made very cautiously; as an
introduction to the problems of so-called mosaic development we shall
consider here new experiments on Calliphora eggs (Nitschmann, 1959,
1961) which have revealed a certain degree of epigenetic determination
as far as location and size of primary blastemas and metameres are
concerned.
In Calliphora (Fig. 8a-g, u), the cleavage energids move from the
cleavage centre (78%) towards the interior of the egg (a, 64 nuclei) and
thence spread so as to enter all surface regions simultaneously (b).
Primary vitellophags lagging behind deteriorate soon after formation of
secondary periplasm (c). The oosome material is taken up by the pole
cells and may be tracked down to the larval gonads. Preblastoderm cells
from both egg poles enter the yolk plasmodium and serve as secondary
vitellophags (c, d). Blastoderm cells from the more equatorial regions,
however, can become supernumerary secondary vitellophags provided
they make contact with the axial endoplasm as is the case after pole
cautery (h) or after complete constriction of the egg (i). Segregation of
metameric blastoderm regions begins ventrally (d, e) by appearance of
the so-called head fold (HF) and anterior as well as posterior border folds
(BF). Dorsally between these, there appear several transverse folds (u).
These are not merely mechanical consequences of germ band extension
upon the dorsal side (cf. Hathaway and Selman, 1961): The folds are
foreshadowed by active inward extension of the cells concerned and are
there before germ band extension; in constricted eggs (t) they form also
in front of the ligature. From the results of ligations during the dorsal
fold stage (u) Nitschmann constructed a plan of metameric regions (v).
The presumptive endoderm appears to be located near the egg poles (e);
its blastoderm cells are distinguished by inclusion of many yolk bodies.
In contrast to the events in Apis (Fig. 3), anterior and posterior endoderm rudiments segregate and invaginate at the same time, and no
supernumery endoderm rudiments are formed in front of or behind a
ligature. A ventral strip of blastoderm containing few yolk bodies differentiates to form the mesoderm. The presumptive ectoderm cells are free
of yolk. Each one of the two endoderm rudiments is capable of developing into a complete mid-gut; mesoderm and ectoderm, however, cannot
replace each other. The mesoderm simultaneously starts to invaginate
near the head fold and in front of the posterior border fold (e); the
dynamic cooperation of these two initial regions of mesoderm invagination has been studied by Nitschmann by means of cautery and ligation
(k-o). Following complete or almost complete ligation, each of the
centres can govern a bigger region than in normal development; the
posterior centre acts earlier (arrows at the right of Fig. 8, 1, m). In
reduced egg systems, one initial region may suffice for invagination of
293
differentiation in the ectoplasm should be made very cautiously; as an
introduction to the problems of so-called mosaic development we shall
consider here new experiments on Calliphora eggs (Nitschmann, 1959,
1961) which have revealed a certain degree of epigenetic determination
as far as location and size of primary blastemas and metameres are
concerned.
In Calliphora (Fig. 8a-g, u), the cleavage energids move from the
cleavage centre (78%) towards the interior of the egg (a, 64 nuclei) and
thence spread so as to enter all surface regions simultaneously (b).
Primary vitellophags lagging behind deteriorate soon after formation of
secondary periplasm (c). The oosome material is taken up by the pole
cells and may be tracked down to the larval gonads. Preblastoderm cells
from both egg poles enter the yolk plasmodium and serve as secondary
vitellophags (c, d). Blastoderm cells from the more equatorial regions,
however, can become supernumerary secondary vitellophags provided
they make contact with the axial endoplasm as is the case after pole
cautery (h) or after complete constriction of the egg (i). Segregation of
metameric blastoderm regions begins ventrally (d, e) by appearance of
the so-called head fold (HF) and anterior as well as posterior border folds
(BF). Dorsally between these, there appear several transverse folds (u).
These are not merely mechanical consequences of germ band extension
upon the dorsal side (cf. Hathaway and Selman, 1961): The folds are
foreshadowed by active inward extension of the cells concerned and are
there before germ band extension; in constricted eggs (t) they form also
in front of the ligature. From the results of ligations during the dorsal
fold stage (u) Nitschmann constructed a plan of metameric regions (v).
The presumptive endoderm appears to be located near the egg poles (e);
its blastoderm cells are distinguished by inclusion of many yolk bodies.
In contrast to the events in Apis (Fig. 3), anterior and posterior endoderm rudiments segregate and invaginate at the same time, and no
supernumery endoderm rudiments are formed in front of or behind a
ligature. A ventral strip of blastoderm containing few yolk bodies differentiates to form the mesoderm. The presumptive ectoderm cells are free
of yolk. Each one of the two endoderm rudiments is capable of developing into a complete mid-gut; mesoderm and ectoderm, however, cannot
replace each other. The mesoderm simultaneously starts to invaginate
near the head fold and in front of the posterior border fold (e); the
dynamic cooperation of these two initial regions of mesoderm invagination has been studied by Nitschmann by means of cautery and ligation
(k-o). Following complete or almost complete ligation, each of the
centres can govern a bigger region than in normal development; the
posterior centre acts earlier (arrows at the right of Fig. 8, 1, m). In
reduced egg systems, one initial region may suffice for invagination of