THE EARLY EMBRYOGENESIS OF INSECTS
269
stress the physiological aspect of such regions and therefore call them
'control centres' (Counce, 1961), using this term to cover the varying
kinds of centres detected since Spemann established the organization
centre. Both initial regions and factor regions are expected to owe their
properties to special substances or structures which, if not known at
present, ultimately may be revealed by improved methods. Since our
knowledge concerning control centres in different egg types is fragmentary, we shall venture occasionally to take peculiarities of an egg region
in substance or structure as signs of its physiological importance, provided this is warranted by observations in other species.
In the egg of Wachtliella (Section I I I B) one control centre is located
near the anterior pole. It contains the zygote nucleus and with it the
genome of the egg. It may be called the cleavage centre (CC) ('Furchungszentrum' of Krause, 1938a) since the cleavage energids start from
there and waves of cleavage mitoses commence there. In Gryllus,
cleavage without nuclei (see Section V B) revealed that the cleavage
centre there is characterized by invisible ooplasmic gradients influencing
the formation of plasma islands during cleavage. In this species, some
structural peculiarities of the centre are well visible before cleavage (Fig.
7 a x ). The first polar body (PB X ) is found on the dorsal ( = animal) egg
side. The second (PB 2 ) is given off by the future egg nucleus during its
migration towards the central region. The zygote nucleus (N) is situated
ventrally in the vicinity of a slight bend in the axial strand of endoplasm
probably originating as protrusion of the perinuclear ergastoplasm in
the cylinder-stage oocyte (cf. Tachycines; v. Kraft, 1960b); the bend is
due most likely to the dorsal migration of the oocyte nucleus before
meiosis. In other insects, where no such structural peculiarities of the
cleavage centre have been described as yet, its physiological activity
may be clearly demonstrated by extensive migration of the female
pronucleus towards the well-defined region where cleavage is to start (e.g.
Pyrilla; Sander, 1956).
Another control centre in the egg of Wachtliella may be situated in
the posterior pole, distinguished by pole plasm and germ cell segregation.
From its functions in some other insects, such a centre near the posterior pole is known as an activation centre (AC) ('Bildungszentrum';
Seidel, 1929; cf. Section VA).
A third control centre, together with the two centres mentioned
above, can be studied in Apis. It was called the differentiation centre
(DC) by Seidel (1924), who recognized it as an initial region in Pyrrhocoris. In the Apis egg (Fig. 3) Schnetter (1934a) found that at a level
which was 76% of the total distance from the posterior pole the following were maximal: the reticuloplasm and periplasm (curve I I ) ; the
affinity for thionine in the 3 2-nuclei stage; mitoses; the number of plasma
269
stress the physiological aspect of such regions and therefore call them
'control centres' (Counce, 1961), using this term to cover the varying
kinds of centres detected since Spemann established the organization
centre. Both initial regions and factor regions are expected to owe their
properties to special substances or structures which, if not known at
present, ultimately may be revealed by improved methods. Since our
knowledge concerning control centres in different egg types is fragmentary, we shall venture occasionally to take peculiarities of an egg region
in substance or structure as signs of its physiological importance, provided this is warranted by observations in other species.
In the egg of Wachtliella (Section I I I B) one control centre is located
near the anterior pole. It contains the zygote nucleus and with it the
genome of the egg. It may be called the cleavage centre (CC) ('Furchungszentrum' of Krause, 1938a) since the cleavage energids start from
there and waves of cleavage mitoses commence there. In Gryllus,
cleavage without nuclei (see Section V B) revealed that the cleavage
centre there is characterized by invisible ooplasmic gradients influencing
the formation of plasma islands during cleavage. In this species, some
structural peculiarities of the centre are well visible before cleavage (Fig.
7 a x ). The first polar body (PB X ) is found on the dorsal ( = animal) egg
side. The second (PB 2 ) is given off by the future egg nucleus during its
migration towards the central region. The zygote nucleus (N) is situated
ventrally in the vicinity of a slight bend in the axial strand of endoplasm
probably originating as protrusion of the perinuclear ergastoplasm in
the cylinder-stage oocyte (cf. Tachycines; v. Kraft, 1960b); the bend is
due most likely to the dorsal migration of the oocyte nucleus before
meiosis. In other insects, where no such structural peculiarities of the
cleavage centre have been described as yet, its physiological activity
may be clearly demonstrated by extensive migration of the female
pronucleus towards the well-defined region where cleavage is to start (e.g.
Pyrilla; Sander, 1956).
Another control centre in the egg of Wachtliella may be situated in
the posterior pole, distinguished by pole plasm and germ cell segregation.
From its functions in some other insects, such a centre near the posterior pole is known as an activation centre (AC) ('Bildungszentrum';
Seidel, 1929; cf. Section VA).
A third control centre, together with the two centres mentioned
above, can be studied in Apis. It was called the differentiation centre
(DC) by Seidel (1924), who recognized it as an initial region in Pyrrhocoris. In the Apis egg (Fig. 3) Schnetter (1934a) found that at a level
which was 76% of the total distance from the posterior pole the following were maximal: the reticuloplasm and periplasm (curve I I ) ; the
affinity for thionine in the 3 2-nuclei stage; mitoses; the number of plasma