VITELLINE MEMBRANE AND CORTICAL PARTICLES 301
cavity formation involves. There are still traces of the hollows that
remain after the exudation of the lamellae from the opened cortical
particles; the extralamellar bodies are stretched tangentially and as a
consequence tightly pressed against the cytoplasmic surface. This change
involves a wide opening-up of the cortical hollows which immediately
after the formation of the stomata are urn-shaped. This wide opening-up
favors a more complete release of the content of the cortical hollows. In
this state the acid formation (Section II,B) undergoes an increase
(Ohnishi and Sugiyama, 1963) which may indicate an increased delivery
of acid mucopolysaccharides into the perivitelline space.
An essental point in the concavity formation is the already mentioned
retraction of the villi from their contact with the fertilization membrane
—a contact that still prevails during the initial state of elevation of
the fertilization membrane. The retraction of the villi is strongly delayed
after pretreatment with ribonuclease in sea water and fertilization in
this medium (Runnström, 1962, Figs. 6-13). Under these conditions a
concavity develops slowly and may be present even 10-15 min after
fertilization. It may then be seen that the sperm aster mainly develops
in the wall of the concavity. This means that the material for the sperm
aster, which consists of adhering endoplasmic vesicles, is displaced in
the direction of the proximal concavity, as is also confirmed by electronmicroscopic studies.
The smoothening, i.e., the retraction of villi from the contact with the
fertilization membrane, involves also deep-going changes in the fine
structure of the fertilization membrane. Before retraction the lamellar
part of the fertilization membrane has a compartmented fine structure of
the kind that has been described above for the filaments of the delayed
lamellae; after retraction of the villi, the periodic fine structure is
obscured by a rather electron-dense substance. This may explain why
the fertilization membrane becomes darker in phase contrast. On the
other hand, the filling material does not influence the birefringence of
the membrane within the rather great errors of the method. The question about the origin of the filling substance cannot yet be answered
definitely. The greater darkness could possibly be explained by referring
to an increasing number of linkages in the lamellae after the concavity
formation. A gradual increase in "darkness" would then be expected, but
the effect came on suddenly and was soon complete, as phase-contrast
observations showed. Furthermore, the phenomenon is strictly dependent
on the retraction of the villi and the wide opening of the cortical hollows.
Instructive were the observations on different regions of the same egg
in the electron microscope ; in one region the villi may be retracted from
the fertilization membrane; this was electron dense without resolvable
cavity formation involves. There are still traces of the hollows that
remain after the exudation of the lamellae from the opened cortical
particles; the extralamellar bodies are stretched tangentially and as a
consequence tightly pressed against the cytoplasmic surface. This change
involves a wide opening-up of the cortical hollows which immediately
after the formation of the stomata are urn-shaped. This wide opening-up
favors a more complete release of the content of the cortical hollows. In
this state the acid formation (Section II,B) undergoes an increase
(Ohnishi and Sugiyama, 1963) which may indicate an increased delivery
of acid mucopolysaccharides into the perivitelline space.
An essental point in the concavity formation is the already mentioned
retraction of the villi from their contact with the fertilization membrane
—a contact that still prevails during the initial state of elevation of
the fertilization membrane. The retraction of the villi is strongly delayed
after pretreatment with ribonuclease in sea water and fertilization in
this medium (Runnström, 1962, Figs. 6-13). Under these conditions a
concavity develops slowly and may be present even 10-15 min after
fertilization. It may then be seen that the sperm aster mainly develops
in the wall of the concavity. This means that the material for the sperm
aster, which consists of adhering endoplasmic vesicles, is displaced in
the direction of the proximal concavity, as is also confirmed by electronmicroscopic studies.
The smoothening, i.e., the retraction of villi from the contact with the
fertilization membrane, involves also deep-going changes in the fine
structure of the fertilization membrane. Before retraction the lamellar
part of the fertilization membrane has a compartmented fine structure of
the kind that has been described above for the filaments of the delayed
lamellae; after retraction of the villi, the periodic fine structure is
obscured by a rather electron-dense substance. This may explain why
the fertilization membrane becomes darker in phase contrast. On the
other hand, the filling material does not influence the birefringence of
the membrane within the rather great errors of the method. The question about the origin of the filling substance cannot yet be answered
definitely. The greater darkness could possibly be explained by referring
to an increasing number of linkages in the lamellae after the concavity
formation. A gradual increase in "darkness" would then be expected, but
the effect came on suddenly and was soon complete, as phase-contrast
observations showed. Furthermore, the phenomenon is strictly dependent
on the retraction of the villi and the wide opening of the cortical hollows.
Instructive were the observations on different regions of the same egg
in the electron microscope ; in one region the villi may be retracted from
the fertilization membrane; this was electron dense without resolvable