12
CHR. P. RAVEN
that various enzymes, vitamin C, and similar substances important for
cell metabolism are also accumulated in layers during centrifuging
(Ries, 1938;Peltrera, 1940; Raven and Bretschneider, 1942).
The normal development of centrifuged eggs became understandable
when it appeared that the stratification brought about by centrifuging
in many cases disappeared in a relatively short time, and a more or less
normal distribution of substances was again attained (Morgan, 1933;
Pasteels, 1934 ; Raven, 1938 ; Ries, 1938 ; Raven and Bretschneider, 1942).
Two aspects must be distinguished in this redistribution of substances :
(1) the disappearance of the stratification and the recovery of a more
homogeneous distribution of substances; (2) the occurrence of ooplasmic
segregations resembling those in normal development.
The stratification of egg substances, brought about by centrifugal
force, becomes a very 'improbable' distribution, as soon as this force
ceases to work. After the end of centrifuging there will therefore be a
tendency to the 'spontaneous' recovery of a more uniform distribution.
The thermal movements of molecules, expressing themselves as diffusion
of dissolved substances and Brownian movement of granules, will
automatically lead to a disappearance of the sharp boundaries between
substances. In Limnaea the substances demonstrable by cytochemical
methods disperse through the egg even more rapidly than the particulate
inclusions (Raven and Bretschneider, 1942). Moreover, all displacements
of substances due to other causes will in general only hasten the attainment of the state of equilibrium, which (in the absence of other forces) is
the homogeneous distribution of substances. Such displacements are
caused, e.g., by the formation and movements of asters, spindles and
nuclei during maturation, amphimixis and cleavage ; by the cytoplasmic
currents occurring during these processes ; by changes in shape of the
egg (amoeboid movements, formation and withdrawal of polar lobes,
formation of cleavage furrows, rounding-off and subsequent flattening
of cleavage cells) taking place at these stages. Moreover, the elastic
recovery of spherical shape by the elongated egg after removal from the
centrifuge (Fig. 4) may play an important part, as has been emphasized
by Peltrera (1940). All this tends to help the disappearance of the
stratification and the recovery of a more homogeneous substance
distribution.
However, the second aspect distinguished above, the occurrence of
ooplasmic segregation, is in a way the reverse of the first one. Here a
greater degree of heterogeneity arises from a more homogeneous
distribution. This represents the transition from a more to a less
'probable' state, unless special forces are admitted driving the system in
this direction. This means, in other words, that the observed phenomena
of segregation point to the existence of such forces.
CHR. P. RAVEN
that various enzymes, vitamin C, and similar substances important for
cell metabolism are also accumulated in layers during centrifuging
(Ries, 1938;Peltrera, 1940; Raven and Bretschneider, 1942).
The normal development of centrifuged eggs became understandable
when it appeared that the stratification brought about by centrifuging
in many cases disappeared in a relatively short time, and a more or less
normal distribution of substances was again attained (Morgan, 1933;
Pasteels, 1934 ; Raven, 1938 ; Ries, 1938 ; Raven and Bretschneider, 1942).
Two aspects must be distinguished in this redistribution of substances :
(1) the disappearance of the stratification and the recovery of a more
homogeneous distribution of substances; (2) the occurrence of ooplasmic
segregations resembling those in normal development.
The stratification of egg substances, brought about by centrifugal
force, becomes a very 'improbable' distribution, as soon as this force
ceases to work. After the end of centrifuging there will therefore be a
tendency to the 'spontaneous' recovery of a more uniform distribution.
The thermal movements of molecules, expressing themselves as diffusion
of dissolved substances and Brownian movement of granules, will
automatically lead to a disappearance of the sharp boundaries between
substances. In Limnaea the substances demonstrable by cytochemical
methods disperse through the egg even more rapidly than the particulate
inclusions (Raven and Bretschneider, 1942). Moreover, all displacements
of substances due to other causes will in general only hasten the attainment of the state of equilibrium, which (in the absence of other forces) is
the homogeneous distribution of substances. Such displacements are
caused, e.g., by the formation and movements of asters, spindles and
nuclei during maturation, amphimixis and cleavage ; by the cytoplasmic
currents occurring during these processes ; by changes in shape of the
egg (amoeboid movements, formation and withdrawal of polar lobes,
formation of cleavage furrows, rounding-off and subsequent flattening
of cleavage cells) taking place at these stages. Moreover, the elastic
recovery of spherical shape by the elongated egg after removal from the
centrifuge (Fig. 4) may play an important part, as has been emphasized
by Peltrera (1940). All this tends to help the disappearance of the
stratification and the recovery of a more homogeneous substance
distribution.
However, the second aspect distinguished above, the occurrence of
ooplasmic segregation, is in a way the reverse of the first one. Here a
greater degree of heterogeneity arises from a more homogeneous
distribution. This represents the transition from a more to a less
'probable' state, unless special forces are admitted driving the system in
this direction. This means, in other words, that the observed phenomena
of segregation point to the existence of such forces.