DEVELOPMENT OF GASTROPODS
11
maturation or early cleavage colour differences between the animal and
vegetal sides often appeared. For instance, in the egg of Columbella the
animal zone of clear cytoplasm stains with a tint corresponding to an
alkaline reaction, then follows a zone with small granules showing a
somewhat more acid reaction, while the region of big yolk platelets in
the vegetal half has a colour indicating a strong acid reaction (Spek,
1934). Similar differences in staining occur in Aplysia (Ries and Gersch,
1936; Raven, 1938).
These phenomena were explained by Spek by assuming that a
segregation of positively and negatively charged colloid particles, which
were at first uniformly mixed throughout the cytoplasm, to opposite
poles of the egg had taken place. This was supposed to be due to the
establishment of an electric field in the egg by differential penetration of
the ions of the outer medium through different parts of the egg cortex.
The electrically charged particles of the cytoplasm then begin to move
electrophoretically in this field. A similar explanation has been given by
Costello (1945), who puts forward the possible role of diffusion potentials
as a cause of ooplasmic segregation.
An attempt to give a direct experimental proof of the action of such
intracellular electric fields was unsuccessful. Limnaea eggs exposed to a
strong external electric field showed no deviations of development,
provided that the egg cortex remained intact ; if, however, after some
time the cortex was destroyed, which always occurred at the side of the
anode, this was followed by a sudden stratification of the egg contents
(Raven, 1948). This shows that the components of the egg cytoplasm are
indeed electrically charged. It appears possible that an intracellular field
is involved in their normal distribution, but that it is insulated from
outward disturbance by the intact egg cortex.
Indirectly, the results of centrifuge experiments may contribute to
our knowledge of the causes of ooplasmic segregation.
It has long been known that in many cases, when the distribution of
egg substances was greatly modified by centrifuging of eggs at the
uncleaved stage, a large proportion of normal embryos was nevertheless
produced (e.g., in Physa and Limnaea:
Conklin, 1910; Raven and
Bretschneider, 1942 ; Aplysia : Peltrera, 1940). As long as it was assumed
that the substances displaced by centrifugal force remained in their new
positions, hardly any redistribution of substances taking place (Conklin,
1910), it was difficult to reconcile this fact with the classical views on
'mosaic development' in molluscs. At first it was explained by the
assumption that the visible egg inclusions displaced by centrifugal force
play no part in the determination of the cells, but are only indifferent
building and food materials. This explanation could hardly be upheld,
however, when it was demonstrated by means of cytochemical methods
11
maturation or early cleavage colour differences between the animal and
vegetal sides often appeared. For instance, in the egg of Columbella the
animal zone of clear cytoplasm stains with a tint corresponding to an
alkaline reaction, then follows a zone with small granules showing a
somewhat more acid reaction, while the region of big yolk platelets in
the vegetal half has a colour indicating a strong acid reaction (Spek,
1934). Similar differences in staining occur in Aplysia (Ries and Gersch,
1936; Raven, 1938).
These phenomena were explained by Spek by assuming that a
segregation of positively and negatively charged colloid particles, which
were at first uniformly mixed throughout the cytoplasm, to opposite
poles of the egg had taken place. This was supposed to be due to the
establishment of an electric field in the egg by differential penetration of
the ions of the outer medium through different parts of the egg cortex.
The electrically charged particles of the cytoplasm then begin to move
electrophoretically in this field. A similar explanation has been given by
Costello (1945), who puts forward the possible role of diffusion potentials
as a cause of ooplasmic segregation.
An attempt to give a direct experimental proof of the action of such
intracellular electric fields was unsuccessful. Limnaea eggs exposed to a
strong external electric field showed no deviations of development,
provided that the egg cortex remained intact ; if, however, after some
time the cortex was destroyed, which always occurred at the side of the
anode, this was followed by a sudden stratification of the egg contents
(Raven, 1948). This shows that the components of the egg cytoplasm are
indeed electrically charged. It appears possible that an intracellular field
is involved in their normal distribution, but that it is insulated from
outward disturbance by the intact egg cortex.
Indirectly, the results of centrifuge experiments may contribute to
our knowledge of the causes of ooplasmic segregation.
It has long been known that in many cases, when the distribution of
egg substances was greatly modified by centrifuging of eggs at the
uncleaved stage, a large proportion of normal embryos was nevertheless
produced (e.g., in Physa and Limnaea:
Conklin, 1910; Raven and
Bretschneider, 1942 ; Aplysia : Peltrera, 1940). As long as it was assumed
that the substances displaced by centrifugal force remained in their new
positions, hardly any redistribution of substances taking place (Conklin,
1910), it was difficult to reconcile this fact with the classical views on
'mosaic development' in molluscs. At first it was explained by the
assumption that the visible egg inclusions displaced by centrifugal force
play no part in the determination of the cells, but are only indifferent
building and food materials. This explanation could hardly be upheld,
however, when it was demonstrated by means of cytochemical methods