THE EARLY EMBRYOGENESIS OF INSECTS
275
5. Heterochronous mitoses: differentiation (presumably endomitotic)
in vitellophags, serosa, and amnion cells; mesoderm segregation;
segmentation except in head lobes and in abdominal segment formation
zone. Morphogenetic movement and blastemic segregation in phases 4
and 5 are controlled presumably by the yolk plasmodium and differentiation centres.
6. Segmentation of abdomen, anatrepsis, and yolk cleavage terminate
reactions at the plasmodial level.
In the following subsections we shall compare the behaviour of cleavage energids in normal eggs of extremely different types, and mention
experiments shedding light upon the mechanism of cleavage.
A. Synchronous Mitoses and Collective Migration of Energids
In the eggs of typical Spiralia, such as molluscs, blastomeres loaded
with yolk cleave later than those containing little or no yolk. Asynchronous cleavage divisions therefore have been assumed to be due to the
presence of a large amount of yolk. In a way, this heterochrony of cell
divisions is even more extreme in insects, where the yolk is frequently
subdivided into cells, if this occurs at all, only after the blastoderm cells
number several hundred. Differences in mitotic rhythm of future blastoderm cells and of vitellophags, as observed for example in Drosophila,
cannot be ascribed however to the deutoplasmic inclusions as such,
because in the yolkfree egg of Apiomorpha, synchrony of cleavage
divisions has already vanished after the third cleavage (Büchner, 1957).
Therefore one may assume that, in superficial cleavage of the insect egg,
the nuclei possess a species-specific division rhythm which is modified
only when the nucleus encounters some specific condition in the ooplasm
(Fig. 2a 5 ). During early cleavage, the energids are equipotent (Seidel,
1932; Ewest, 1937; Maschlanka, 1938; v. Borstel, 1957; and others).
During this period, the nuclei apparently do not influence development,
but rather are 'objects of plasma factors' in Boveri's sense (v. Borstel,
1957; Reinhardt, 1960; Sander, 1959-1962). Waves of mitoses normally
start from the cleavage centre (CC) (Gryllus, Fig. 7a 3 ; Notonecta, Fig.
4c x ) and may persist despite centrifugation (Wachtliella; Geyer-Duszynska, 1959); but in some instances mitoses after IVth cleavage start in
the DC (Apis, Fig. 3).
In total cleavage as well as in superficial cleavage there are exceptions
from Sachs' rule which states that the spindles of each cleavage mitosis
are situated at right angles to those of the previous division. In insects,
this rule at best is valid only for the first few mitoses leading to a swarm
of energids; thereafter, the degrees of freedom of spindle orientation
may be reduced to 2 or even 1 directions only. What are the factors
orientating the spindles and guiding energid migration?
275
5. Heterochronous mitoses: differentiation (presumably endomitotic)
in vitellophags, serosa, and amnion cells; mesoderm segregation;
segmentation except in head lobes and in abdominal segment formation
zone. Morphogenetic movement and blastemic segregation in phases 4
and 5 are controlled presumably by the yolk plasmodium and differentiation centres.
6. Segmentation of abdomen, anatrepsis, and yolk cleavage terminate
reactions at the plasmodial level.
In the following subsections we shall compare the behaviour of cleavage energids in normal eggs of extremely different types, and mention
experiments shedding light upon the mechanism of cleavage.
A. Synchronous Mitoses and Collective Migration of Energids
In the eggs of typical Spiralia, such as molluscs, blastomeres loaded
with yolk cleave later than those containing little or no yolk. Asynchronous cleavage divisions therefore have been assumed to be due to the
presence of a large amount of yolk. In a way, this heterochrony of cell
divisions is even more extreme in insects, where the yolk is frequently
subdivided into cells, if this occurs at all, only after the blastoderm cells
number several hundred. Differences in mitotic rhythm of future blastoderm cells and of vitellophags, as observed for example in Drosophila,
cannot be ascribed however to the deutoplasmic inclusions as such,
because in the yolkfree egg of Apiomorpha, synchrony of cleavage
divisions has already vanished after the third cleavage (Büchner, 1957).
Therefore one may assume that, in superficial cleavage of the insect egg,
the nuclei possess a species-specific division rhythm which is modified
only when the nucleus encounters some specific condition in the ooplasm
(Fig. 2a 5 ). During early cleavage, the energids are equipotent (Seidel,
1932; Ewest, 1937; Maschlanka, 1938; v. Borstel, 1957; and others).
During this period, the nuclei apparently do not influence development,
but rather are 'objects of plasma factors' in Boveri's sense (v. Borstel,
1957; Reinhardt, 1960; Sander, 1959-1962). Waves of mitoses normally
start from the cleavage centre (CC) (Gryllus, Fig. 7a 3 ; Notonecta, Fig.
4c x ) and may persist despite centrifugation (Wachtliella; Geyer-Duszynska, 1959); but in some instances mitoses after IVth cleavage start in
the DC (Apis, Fig. 3).
In total cleavage as well as in superficial cleavage there are exceptions
from Sachs' rule which states that the spindles of each cleavage mitosis
are situated at right angles to those of the previous division. In insects,
this rule at best is valid only for the first few mitoses leading to a swarm
of energids; thereafter, the degrees of freedom of spindle orientation
may be reduced to 2 or even 1 directions only. What are the factors
orientating the spindles and guiding energid migration?