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
271
initial region, which is correlated with the pro thorax. By observing the
development of posterior isolates, Schnetter (1936) was able to
prove that the locus of this initial region is not fixed despite its ooplasmic
preformation (Fig. 3, below). Following ligation above the 76% level
during the early blastoderm (12 hr), the posterior isolate forms a dwarf
larva. In this case, a new maximum of cell number and secondary
periplasm appears at the 60% level and it is there that yolk contraction
(f), formation of germ layers, segmentation, and organ formation begin.
The egg, or at least its anterior portion, behaves like a blastema field;
the differentiation centre is the centre of an ooplasmically preformed
morphogenetic field. The material located there is merely predisposed
for labium and pro thorax, but not yet determined.
In the differentiated blastoderm (24 hr, Fig. 3c), the segregation of a
metameric pattern (VI) must have taken place, because ligation results
in the production of complementary larval parts except that, in front of
and behind the ligation, supernumerary endoderm rudiments are formed
instead of mesoderm (h).
We may ask now how the locus of the differentiation centre is determined in the blastoderm. E. Sauer (1954) was able to show that at 12 hr
the differentiation of the middle plate into endoderm or mesoderm is
dependent on factors located at or behind the 36% level. Without these
factors, the whole middle plate of anterior isolates will turn into endoderm. The mesoderm activation factors spread anteriorly in the egg and
have reached the preformed differentiation centre 24 hr after oviposition
(Fig. 3, curve V). In this way, the centre might be activated and its
locus thus determined.
Origin, speed of migration, and other properties of these mesoderm
activation factors are not yet known. However, Schnetter (1934a) had
demonstrated a second, but lower, maximum of periplasm thickness and,
later on, of blastoderm cell number at the 36% level (curves II and IV)
which may indicate an abdominal centre. We prefer to call this an
activation centre (AC) rather than an abdominal DC (F. Seidel, 1960)
since, unlike the prothoracic DC, it is not the initial region of mesoderm
differentiation; however, its homologies are not yet clear.
Contrary to the differentiation and activation centres, the cleavage
centre situated at the 90% level is not ooplasmically preformed apart
from the maturation plasm (Fig. 3, curve I). By mitotic movement, the
nuclei in the axial plasm form a group which as a whole moves backwards into the DC (curve III). Thence they spread along the longitudinal axis until they form a club-shaped spheroid around the primary
vitellophags located in the DC (b). The differences in cleavage control
which are shown by haploid and diploid eggs of Apis will be described
later (Section IV A).
271
initial region, which is correlated with the pro thorax. By observing the
development of posterior isolates, Schnetter (1936) was able to
prove that the locus of this initial region is not fixed despite its ooplasmic
preformation (Fig. 3, below). Following ligation above the 76% level
during the early blastoderm (12 hr), the posterior isolate forms a dwarf
larva. In this case, a new maximum of cell number and secondary
periplasm appears at the 60% level and it is there that yolk contraction
(f), formation of germ layers, segmentation, and organ formation begin.
The egg, or at least its anterior portion, behaves like a blastema field;
the differentiation centre is the centre of an ooplasmically preformed
morphogenetic field. The material located there is merely predisposed
for labium and pro thorax, but not yet determined.
In the differentiated blastoderm (24 hr, Fig. 3c), the segregation of a
metameric pattern (VI) must have taken place, because ligation results
in the production of complementary larval parts except that, in front of
and behind the ligation, supernumerary endoderm rudiments are formed
instead of mesoderm (h).
We may ask now how the locus of the differentiation centre is determined in the blastoderm. E. Sauer (1954) was able to show that at 12 hr
the differentiation of the middle plate into endoderm or mesoderm is
dependent on factors located at or behind the 36% level. Without these
factors, the whole middle plate of anterior isolates will turn into endoderm. The mesoderm activation factors spread anteriorly in the egg and
have reached the preformed differentiation centre 24 hr after oviposition
(Fig. 3, curve V). In this way, the centre might be activated and its
locus thus determined.
Origin, speed of migration, and other properties of these mesoderm
activation factors are not yet known. However, Schnetter (1934a) had
demonstrated a second, but lower, maximum of periplasm thickness and,
later on, of blastoderm cell number at the 36% level (curves II and IV)
which may indicate an abdominal centre. We prefer to call this an
activation centre (AC) rather than an abdominal DC (F. Seidel, 1960)
since, unlike the prothoracic DC, it is not the initial region of mesoderm
differentiation; however, its homologies are not yet clear.
Contrary to the differentiation and activation centres, the cleavage
centre situated at the 90% level is not ooplasmically preformed apart
from the maturation plasm (Fig. 3, curve I). By mitotic movement, the
nuclei in the axial plasm form a group which as a whole moves backwards into the DC (curve III). Thence they spread along the longitudinal axis until they form a club-shaped spheroid around the primary
vitellophags located in the DC (b). The differences in cleavage control
which are shown by haploid and diploid eggs of Apis will be described
later (Section IV A).