T H E E A R L Y E M B R Y O G E N E S I S O F I N S E C T S
267
Wachtliella persicaria (Geyer-Duszynska, 1959) (Fig. 2, a-k). The eggs
of this species contain a thin periplasm, fine meshes of reticuloplasm,
and rather homogenous plasma islands. Near the posterior pole the
meshes of the reticuloplasm are big and filled with a homogenous
substance staining darkly with Heidenhain's iron haematoxylin (a x ).
Mitosis starts near the anterior pole; one of the issuing 8 nuclei migrates
into the pole plasm (a 4 ) to become the nucleus of the mother germ cell
which has its own rhythm of cleavage (a 5 ) and gives rise to the pole cells
(PC, a 7 ). During IVth cleavage mitosis, the 7 other nuclei eliminate
some 30-odd chromosomes each. Thereafter, 7 Feulgen-positive bodies
(not shown in Fig. 2) are found to degenerate close to the longitudinal
axis of the egg. The remaining 14 somatic nuclei contain only 8 chromosomes each. They form the blastoderm (a 7 ) in which, during V l l t h
cleavage mitosis, two sex chromosomes are eliminated in the male eggs
(a 6 ). From these observations one may expect elimination factors for
somatic differentiation in the endoplasm, and for sexual differentiation
in the periplasm. What are the factors in the pole plasm preventing
elimination and effecting pole cell formation?
Cautery (Fig. 2 b x _ 2 , destroyed region black), permanent or temporary
subdivision of the egg with a hair ( c ^ ) , U.V.-irradiation {d ± _ 2 , target
area marked) and centrifugation (e-g, arrows indicate direction of
centrifugal force) have been employed to dislocate nuclei within the
somatic region of the egg, or to make them invade the pole plasm at the
wrong time. In all cases the IVth mitosis involved elimination except in
those nuclei which had reached the pole plasm containing the homogenous, darkly staining substance. If the immigration of nuclei into the
pole plasm is delayed by means of a hair (c 2 ), or by U.V.-damaged plasm
(d 2 ), or by killing the hindmost nucleus (d x ), or by low centrifugal force
(g), the posterior pole region produces small mother germ cells with only
8 chromosomes. High centrifugal force shifts the homogenous substance from the pole towards the centre of the egg (h-k). In this case all
nuclei eliminate chromosomes with the exception of those that happen
to be in the vicinity of the translocated homogenous substance deeply
staining with haematoxylin. This substance has thus been proved to
contain the factor preventing chromosome elimination. Since this factor
is now lacking at the posterior pole, nuclei reaching the pole eliminate
chromosomes (h) if they have not already done so (i). Nevertheless, in
both cases small mother germ cells are formed at the posterior pole.
There is, then, a second factor located in the pole plasm governing germ
cell formation. It was not dislocated by strong centrifugation and is
supposed to be found in the reticuloplasm near the pole. Very strong
centrifugation causes big vacuoles to be formed in the orange zone and a
white cap to appear in the posterior pole (k). Under these conditions the
267
Wachtliella persicaria (Geyer-Duszynska, 1959) (Fig. 2, a-k). The eggs
of this species contain a thin periplasm, fine meshes of reticuloplasm,
and rather homogenous plasma islands. Near the posterior pole the
meshes of the reticuloplasm are big and filled with a homogenous
substance staining darkly with Heidenhain's iron haematoxylin (a x ).
Mitosis starts near the anterior pole; one of the issuing 8 nuclei migrates
into the pole plasm (a 4 ) to become the nucleus of the mother germ cell
which has its own rhythm of cleavage (a 5 ) and gives rise to the pole cells
(PC, a 7 ). During IVth cleavage mitosis, the 7 other nuclei eliminate
some 30-odd chromosomes each. Thereafter, 7 Feulgen-positive bodies
(not shown in Fig. 2) are found to degenerate close to the longitudinal
axis of the egg. The remaining 14 somatic nuclei contain only 8 chromosomes each. They form the blastoderm (a 7 ) in which, during V l l t h
cleavage mitosis, two sex chromosomes are eliminated in the male eggs
(a 6 ). From these observations one may expect elimination factors for
somatic differentiation in the endoplasm, and for sexual differentiation
in the periplasm. What are the factors in the pole plasm preventing
elimination and effecting pole cell formation?
Cautery (Fig. 2 b x _ 2 , destroyed region black), permanent or temporary
subdivision of the egg with a hair ( c ^ ) , U.V.-irradiation {d ± _ 2 , target
area marked) and centrifugation (e-g, arrows indicate direction of
centrifugal force) have been employed to dislocate nuclei within the
somatic region of the egg, or to make them invade the pole plasm at the
wrong time. In all cases the IVth mitosis involved elimination except in
those nuclei which had reached the pole plasm containing the homogenous, darkly staining substance. If the immigration of nuclei into the
pole plasm is delayed by means of a hair (c 2 ), or by U.V.-damaged plasm
(d 2 ), or by killing the hindmost nucleus (d x ), or by low centrifugal force
(g), the posterior pole region produces small mother germ cells with only
8 chromosomes. High centrifugal force shifts the homogenous substance from the pole towards the centre of the egg (h-k). In this case all
nuclei eliminate chromosomes with the exception of those that happen
to be in the vicinity of the translocated homogenous substance deeply
staining with haematoxylin. This substance has thus been proved to
contain the factor preventing chromosome elimination. Since this factor
is now lacking at the posterior pole, nuclei reaching the pole eliminate
chromosomes (h) if they have not already done so (i). Nevertheless, in
both cases small mother germ cells are formed at the posterior pole.
There is, then, a second factor located in the pole plasm governing germ
cell formation. It was not dislocated by strong centrifugation and is
supposed to be found in the reticuloplasm near the pole. Very strong
centrifugation causes big vacuoles to be formed in the orange zone and a
white cap to appear in the posterior pole (k). Under these conditions the