THE EARLY EMBRYOGENESIS OF INSECTS
279
nuclei in Panorpa, could not be verified in Culex and Oncopeltus (Krause,
unpublished). A forward movement of the presumptive germ disc as
assumed in the second hypothesis is shown in some instances. In Panorpa,
the germ disc (Fig. 4b 2 ) expands'first anteriorly (Fig. 4b 3 ) and later in a
posterior direction (Fig. 4b 4 ). In Tenebrio, the whole germ disc expands
anteriorly (arrow) in the beginning (Fig. 5a 2 _ 4 ), but later the germ band
exceeds the egg in length (similar to Fig. 5b 4 ); mesoderm formation
begins near the rear end (Fig. 5a 3 , dotted arrow) and the prothoracic DC
appears in front of the former CC! In Melasoma populi a stretching of
the embryonic pattern may have taken place even during early cleavage
(arrows in Fig. 5b 4 ), earlier than in Tenebrio. Presumably extraembryonic periplasm collects near the anterior pole, and at the same time the
disc-shaped oosome near the posterior pole crumples up. In Leptinotarsa
ectoplasm movements may have taken place before oviposition. The
third hypothesis, namely, the existence of a category of insects showing
only the differentiation centre and no activation or cleavage centre, is
maintained by Haget (1953) on the basis of his work on Leptinotarsa.
The examples quoted make evident the influence of egg architecture
upon cleavage. Control centres, gradient systems (animal-vegetal,
anterior-posterior, ecto-endo gradients) and essential elements of egg
architecture (periplasm, marginal and axial endoplasm, oosome, etc.)
may all play a role in cleavage; in varying combinations these elements
constitute the specific ooplasmic control pattern of the different egg
types. During the course of development, the energids pass through two
stages which appear essential for interaction with some of the elements
listed above. The first stage is reached when the swarm of energids
(usually 64) approaches the vicinity of the posterior pole; the second
stage is marked by the movement of energids (usually about 512) from
the spheroid arrangement into the periplasm. Analysis of possible nucleocytoplasmic and inter-ooplasmic reactions occurring during these stages
and during cleavage in general is difficult because usually only germ
parts situated behind each other or at the egg surface may be isolated or
destroyed, and transplantations are hardly possible. The limited knowledge gained so far by analysis of cleavage processes is dealt with in the
following section.
V. Activation
Activation of developmental processes at the ooplasmic reaction level
is analogous to the induction of the development processes on the
blastemic reaction level. For an induction to take place, inductor blastema and competent blastema have to be brought into contact by morphogenetic movements or by the experimenter; in the same way, in the
279
nuclei in Panorpa, could not be verified in Culex and Oncopeltus (Krause,
unpublished). A forward movement of the presumptive germ disc as
assumed in the second hypothesis is shown in some instances. In Panorpa,
the germ disc (Fig. 4b 2 ) expands'first anteriorly (Fig. 4b 3 ) and later in a
posterior direction (Fig. 4b 4 ). In Tenebrio, the whole germ disc expands
anteriorly (arrow) in the beginning (Fig. 5a 2 _ 4 ), but later the germ band
exceeds the egg in length (similar to Fig. 5b 4 ); mesoderm formation
begins near the rear end (Fig. 5a 3 , dotted arrow) and the prothoracic DC
appears in front of the former CC! In Melasoma populi a stretching of
the embryonic pattern may have taken place even during early cleavage
(arrows in Fig. 5b 4 ), earlier than in Tenebrio. Presumably extraembryonic periplasm collects near the anterior pole, and at the same time the
disc-shaped oosome near the posterior pole crumples up. In Leptinotarsa
ectoplasm movements may have taken place before oviposition. The
third hypothesis, namely, the existence of a category of insects showing
only the differentiation centre and no activation or cleavage centre, is
maintained by Haget (1953) on the basis of his work on Leptinotarsa.
The examples quoted make evident the influence of egg architecture
upon cleavage. Control centres, gradient systems (animal-vegetal,
anterior-posterior, ecto-endo gradients) and essential elements of egg
architecture (periplasm, marginal and axial endoplasm, oosome, etc.)
may all play a role in cleavage; in varying combinations these elements
constitute the specific ooplasmic control pattern of the different egg
types. During the course of development, the energids pass through two
stages which appear essential for interaction with some of the elements
listed above. The first stage is reached when the swarm of energids
(usually 64) approaches the vicinity of the posterior pole; the second
stage is marked by the movement of energids (usually about 512) from
the spheroid arrangement into the periplasm. Analysis of possible nucleocytoplasmic and inter-ooplasmic reactions occurring during these stages
and during cleavage in general is difficult because usually only germ
parts situated behind each other or at the egg surface may be isolated or
destroyed, and transplantations are hardly possible. The limited knowledge gained so far by analysis of cleavage processes is dealt with in the
following section.
V. Activation
Activation of developmental processes at the ooplasmic reaction level
is analogous to the induction of the development processes on the
blastemic reaction level. For an induction to take place, inductor blastema and competent blastema have to be brought into contact by morphogenetic movements or by the experimenter; in the same way, in the