10
CHR. P. RAVEN
(1955) observed that in Limnaea eggs centrifuged at a late 4-cell stage
often a supernumerary cephalic plate, giving rise to a cerebral ganglion,
tentacle and eye, was formed in an abnormal location, either in the
pre-trochal or the post-trochal region or on the boundary between the
two. Moreover, supernumerary foregut structures (e.g., radular sac,
salivary glands) may also appear in these embryos (Raven and Tates,
1961). The frequency, and, to a certain extent, the nature of the
malformations produced is dependent upon the direction of the
centrifugal force with respect to the main axis of the egg (Raven and
Tates, 1961).
The most obvious explanation of these facts is given by the assumption that the normal course of development in general, and the
differentiation of various organs and body parts in particular, is
dependent on a normal segregation and distribution of the cytoplasmic
substances of the egg. If, by any interference with the normal process of
ooplasmic segregation, certain substances pass into the 'wrong' cells,
this may lead either to the suppression of certain organs or to their
formation in abnormal places. The cytoplasmic substances therefore
clearly have a morphogenetic significance.
This notion of morphogenetic substances should be used with due
caution, however. It should not be taken too strictly, as if there were a
rectilinear relation between the presence of a single substance and the
differentiation of a certain organ. Eggs may become fragmented by
strong centrifugation. Egg fragments obtained in this way may sometimes develop to more or less normal dwarf larvae (Clement, 1938;
Costello, 1939; Peltrera, 1940; Morrill, 1961). Such fragments may lack
either protein yolk, or fatty yolk, or even both ; fragments consisting
only of hyaloplasm may produce practically normal dwarf larvae.
Peltrera (1940) points out that such larvae show no definite defects, but
only a slightly dysharmonie development with reduction of certain
organs, e.g., underdevelopment of the foot, or failing antero-posterior
elongation. A normal proportion of various cytoplasmic substances is
required for normal development. If this 'histochemical equilibrium' is
broken, the harmonic development of the larvae is disturbed.
IV. The Causality of Ooplasmic Segregation
One of the first attempts to give an explanation of the phenomena of
ooplasmic segregation was made by Spek. He started from observations
on the eggs of various animals treated with vital stains having the
properties of pH-indicators, as e.g., neutral red, Nile blue hydrochloride,
brilliant cresyl violet etc. After such treatment eggs belonging to various
groups of animals were at first uniformly coloured, but at the time of
CHR. P. RAVEN
(1955) observed that in Limnaea eggs centrifuged at a late 4-cell stage
often a supernumerary cephalic plate, giving rise to a cerebral ganglion,
tentacle and eye, was formed in an abnormal location, either in the
pre-trochal or the post-trochal region or on the boundary between the
two. Moreover, supernumerary foregut structures (e.g., radular sac,
salivary glands) may also appear in these embryos (Raven and Tates,
1961). The frequency, and, to a certain extent, the nature of the
malformations produced is dependent upon the direction of the
centrifugal force with respect to the main axis of the egg (Raven and
Tates, 1961).
The most obvious explanation of these facts is given by the assumption that the normal course of development in general, and the
differentiation of various organs and body parts in particular, is
dependent on a normal segregation and distribution of the cytoplasmic
substances of the egg. If, by any interference with the normal process of
ooplasmic segregation, certain substances pass into the 'wrong' cells,
this may lead either to the suppression of certain organs or to their
formation in abnormal places. The cytoplasmic substances therefore
clearly have a morphogenetic significance.
This notion of morphogenetic substances should be used with due
caution, however. It should not be taken too strictly, as if there were a
rectilinear relation between the presence of a single substance and the
differentiation of a certain organ. Eggs may become fragmented by
strong centrifugation. Egg fragments obtained in this way may sometimes develop to more or less normal dwarf larvae (Clement, 1938;
Costello, 1939; Peltrera, 1940; Morrill, 1961). Such fragments may lack
either protein yolk, or fatty yolk, or even both ; fragments consisting
only of hyaloplasm may produce practically normal dwarf larvae.
Peltrera (1940) points out that such larvae show no definite defects, but
only a slightly dysharmonie development with reduction of certain
organs, e.g., underdevelopment of the foot, or failing antero-posterior
elongation. A normal proportion of various cytoplasmic substances is
required for normal development. If this 'histochemical equilibrium' is
broken, the harmonic development of the larvae is disturbed.
IV. The Causality of Ooplasmic Segregation
One of the first attempts to give an explanation of the phenomena of
ooplasmic segregation was made by Spek. He started from observations
on the eggs of various animals treated with vital stains having the
properties of pH-indicators, as e.g., neutral red, Nile blue hydrochloride,
brilliant cresyl violet etc. After such treatment eggs belonging to various
groups of animals were at first uniformly coloured, but at the time of