IX. DEVELOPMENT OF THE TELEOSTEAN EGG
389
the polarity axis being in Lebistes (Vakaet) perpendicular to the direction of the ovarian hilum (Fig. 7) (see, further on, the objections of
Clavert and Filogamo, 1957).
The question arises as to whether nuclear migration, the first
observable manifestation, is really the initial manifestation of polarity
or whether it has already been preceded by an axial organization of the
egg constituents which guides the nucleus in its motion. This cannot be
answered yet.
Is this primary polarity (Harrison, 1945) reinforced afterwards by an
orderly arrangement of the constituents (polarity of stratification of
Harrison, or better still, in this case, of segregation) forming a cytoframework? Considering the changes which take place in the egg during
bipolar differentiation, the existence of an orderly structure of the whole
does not seem very probable. If a structural unity manages to persist,
it can only be in the cortex ('cortical field' of Dalcq, 1941). Pasteéis
(1946), though for different reasons, also comes to the same conclusion
about the egg of Amphibia. The directed contractions observed in the
egg of Brachydanio could be a visible manifestation of the polarized
structure. Perhaps we can suggest with Fauré-Frémiet (1943) than an
initial protoplasmic dissymmetry is the essential factor of membrane
polarity.
Bipolar differentiation reinforces, sooner or later, primary polarity (of
direction) with a final structural polarity. This second phase is determinated quite early in Salmo and Leuciscus and only ends at activation.
When differentiation comes to a standstill within the maternal organism,
the pathways of the cytoplasmic migration are already marked out.
When the movement is resumed all the blastoderm has to retract a kind
of pseudopodia. No possibility is left for exterior causes to modify the
course of these movements because the materials cannot flow any longer
under the influence of gravity (see also McEwen, 1930, on Hemichromis)
as they can in Amphibia.
Such an early and irreversible determination probably does not exist
in all types of eggs. In Brachydanio, no stratification polarity is observable at laying; in Fundulus (Kao and Chambers, 1954b), the place of the
blastoderm can be determined at will by pricking the ripe egg in a NaCl
or KCl solution, though not in sea-water.
2. Bilateral Symmetry
The second factor of anisotropy is bilateral symmetry, and it probably
appears later.
It seems that in the egg of Amphibia, symmetrization takes place in
two stages: a labile symmetry exists before laying and is sometimes
visible from the outside (Pasteéis, 1932). This first symmetry is reinforced
389
the polarity axis being in Lebistes (Vakaet) perpendicular to the direction of the ovarian hilum (Fig. 7) (see, further on, the objections of
Clavert and Filogamo, 1957).
The question arises as to whether nuclear migration, the first
observable manifestation, is really the initial manifestation of polarity
or whether it has already been preceded by an axial organization of the
egg constituents which guides the nucleus in its motion. This cannot be
answered yet.
Is this primary polarity (Harrison, 1945) reinforced afterwards by an
orderly arrangement of the constituents (polarity of stratification of
Harrison, or better still, in this case, of segregation) forming a cytoframework? Considering the changes which take place in the egg during
bipolar differentiation, the existence of an orderly structure of the whole
does not seem very probable. If a structural unity manages to persist,
it can only be in the cortex ('cortical field' of Dalcq, 1941). Pasteéis
(1946), though for different reasons, also comes to the same conclusion
about the egg of Amphibia. The directed contractions observed in the
egg of Brachydanio could be a visible manifestation of the polarized
structure. Perhaps we can suggest with Fauré-Frémiet (1943) than an
initial protoplasmic dissymmetry is the essential factor of membrane
polarity.
Bipolar differentiation reinforces, sooner or later, primary polarity (of
direction) with a final structural polarity. This second phase is determinated quite early in Salmo and Leuciscus and only ends at activation.
When differentiation comes to a standstill within the maternal organism,
the pathways of the cytoplasmic migration are already marked out.
When the movement is resumed all the blastoderm has to retract a kind
of pseudopodia. No possibility is left for exterior causes to modify the
course of these movements because the materials cannot flow any longer
under the influence of gravity (see also McEwen, 1930, on Hemichromis)
as they can in Amphibia.
Such an early and irreversible determination probably does not exist
in all types of eggs. In Brachydanio, no stratification polarity is observable at laying; in Fundulus (Kao and Chambers, 1954b), the place of the
blastoderm can be determined at will by pricking the ripe egg in a NaCl
or KCl solution, though not in sea-water.
2. Bilateral Symmetry
The second factor of anisotropy is bilateral symmetry, and it probably
appears later.
It seems that in the egg of Amphibia, symmetrization takes place in
two stages: a labile symmetry exists before laying and is sometimes
visible from the outside (Pasteéis, 1932). This first symmetry is reinforced