THE EARLY EMBRYOGENESIS OF INSECTS
289
Differenzierungszentrum' of Seidel), like the centre of a morphogenetic
field, may be shifted by regulation. It is connected with peripheral
conditions (topical DC), which cannot be shifted ('physiologisches
Differenzierungszentrum' of Seidel).
In Tachycines, those blastoderm cells which are the first to assemble
dorsally in the region of the cleavage centre, subsequently enter the
head lobes. Before yolk contraction, secondary vitellophags are seen to
enter this region. It is not certain whether these are necessary for the
contraction, or whether there are other peripheral prerequisites for this
process. The procephalic DC of Tachycines might be dynamically preformed in the ooplasmodium and topically in the cortex, as is the prothoracic DC of Platycnemis. Later on it may be taken up by the blastoderm and eventually, as a morphogenetic field centre, gain the faculty of
regulation. The procephalic region of the germ disc of Tachycines continues to lead in differentiation for some time (Krause, 1938b, 1953). It
is only when the primitive groove closes that the thoracic region may
gain the lead.
In Gryllus the centres of the blastoderm cell assemblage also are
situated dorso-laterally near the CC (Krause, 1939a; Sauer, 1961b); the
maximum of latero-medial contraction (DC) is situated before or within
the region of the prospective mandibular segment (Kanellis, 1952; Sauer,
1961a). This procephalic DC appears again as an initial region of mesoderm formation in the head lobes (Mahr, 1960a). The capacity to regulate after lateral U.V.-irradiation seems to decrease in the caudocranial direction (Sauer, 1961b). In the metameric region mesoderm
formation begins simultaneously at the posterior end and in the head
lobes. Thus, instead of a prothoracic DC, we find in Gryllus a system
consisting of a procephalic DC and an abdominal DC. In Notonecta
(Krause, 1958b) and Euscelis (Sander, 1959) the germ anläge first takes
form as lateral thickenings of the blastoderm; soon, however, the head
lobe region is marked by a still thicker blastoderm. Mesoderm imagination starts in the thorax in Notonecta (Fig. 4c 2 ), in the abdominal region
in Tenebrio (Fig. 5a 3 ), and at both ends of the middle plate in Calliphora
(Fig. 8f). Apis, with a smaller larval head, shows a prothoracic DC (Fig.
3d); Pimpla, with a well-developed gnathal region, has its blastemic DC
in the first maxillary segment (Fig. 4d 2 ). In comparison with Tachycines
and Gryllus, the compound eyes of the larva of Platycnemis appear
reduced. These facts indicate that privileges in blastemic differentiation
may also depend upon organization of the larva that is to emerge from
the egg.
One must consider as primitive, in the sense of resembling spiralian
development, that egg type in which ectoderm differentiation starts
dorso-anteriorly, i.e., at the animal pole, whereas mesoderm is formed
289
Differenzierungszentrum' of Seidel), like the centre of a morphogenetic
field, may be shifted by regulation. It is connected with peripheral
conditions (topical DC), which cannot be shifted ('physiologisches
Differenzierungszentrum' of Seidel).
In Tachycines, those blastoderm cells which are the first to assemble
dorsally in the region of the cleavage centre, subsequently enter the
head lobes. Before yolk contraction, secondary vitellophags are seen to
enter this region. It is not certain whether these are necessary for the
contraction, or whether there are other peripheral prerequisites for this
process. The procephalic DC of Tachycines might be dynamically preformed in the ooplasmodium and topically in the cortex, as is the prothoracic DC of Platycnemis. Later on it may be taken up by the blastoderm and eventually, as a morphogenetic field centre, gain the faculty of
regulation. The procephalic region of the germ disc of Tachycines continues to lead in differentiation for some time (Krause, 1938b, 1953). It
is only when the primitive groove closes that the thoracic region may
gain the lead.
In Gryllus the centres of the blastoderm cell assemblage also are
situated dorso-laterally near the CC (Krause, 1939a; Sauer, 1961b); the
maximum of latero-medial contraction (DC) is situated before or within
the region of the prospective mandibular segment (Kanellis, 1952; Sauer,
1961a). This procephalic DC appears again as an initial region of mesoderm formation in the head lobes (Mahr, 1960a). The capacity to regulate after lateral U.V.-irradiation seems to decrease in the caudocranial direction (Sauer, 1961b). In the metameric region mesoderm
formation begins simultaneously at the posterior end and in the head
lobes. Thus, instead of a prothoracic DC, we find in Gryllus a system
consisting of a procephalic DC and an abdominal DC. In Notonecta
(Krause, 1958b) and Euscelis (Sander, 1959) the germ anläge first takes
form as lateral thickenings of the blastoderm; soon, however, the head
lobe region is marked by a still thicker blastoderm. Mesoderm imagination starts in the thorax in Notonecta (Fig. 4c 2 ), in the abdominal region
in Tenebrio (Fig. 5a 3 ), and at both ends of the middle plate in Calliphora
(Fig. 8f). Apis, with a smaller larval head, shows a prothoracic DC (Fig.
3d); Pimpla, with a well-developed gnathal region, has its blastemic DC
in the first maxillary segment (Fig. 4d 2 ). In comparison with Tachycines
and Gryllus, the compound eyes of the larva of Platycnemis appear
reduced. These facts indicate that privileges in blastemic differentiation
may also depend upon organization of the larva that is to emerge from
the egg.
One must consider as primitive, in the sense of resembling spiralian
development, that egg type in which ectoderm differentiation starts
dorso-anteriorly, i.e., at the animal pole, whereas mesoderm is formed