154
R. LALLIER
animalization. The conjoined studies of animalization and vegetalization
processes are indeed essential for the understanding of embryonic
determination and differentiation.
IV. The Gradients
4 A gradient is a system of order involving progressively increasing or
decreasing entities or intensities from one pole or point of a morphogenetic field to another' (Needham, 1950). The interpretations of the
embryonic development of the sea urchin egg are dominated by the
notion of gradient. This notion is derived from morphological and
physiological observations.
According to Boveri (1901) the most vegetative parts exert a determinant control on the other parts of the egg. This control is maximal at
the vegetal pole and it extends, its influence decreasing, towards the
animal pole.
According to Child (1916, 1928, 1936, 1941) a single physiological
gradient controls morphogenesis. This gradient shows a quantitative
decrease along the axis of the organism, the maximum value being
situated at the place where morphogenesis is most active. Child demonstrated the existence of this physiological gradient by diverse processes
(differential susceptibility to toxic agents, reduction during anaerobiosis
of oxidation-reduction indicators).
Runnström (1928a, b, 1929) suggested that there exist two antagonistic
metabolic gradients qualitatively different and overlapping one another.
One of these gradients has its maximum at the animal pole, the other at
the vegetal pole. The interaction between these two gradients would
control the differentiation of the embryo.
Gradients of reduction are observed during the reduction of methylene
blue, or Janus green, by embryos in anaerobiosis. These gradients
indicate a differential distribution of metabolic activities along the
embryonic axis.
Child (1936) has noted during the segmentation of the sea urchin egg,
and in the young blastula, a gradient of reduction decreasing from the
animal pole to the vegetal pole (basipetal gradient). In the advanced
blastula, and during the gastrulation, a second gradient of reduction
appears, directed from the vegetal pole to the animal pole (acropetal
gradient). This acropetal gradient is stronger than the basipetal one and
coexists with it.
Hörstadius (1952, 1955) has confirmed these observations in normal
eggs (Fig. 3). Further, Hörstadius has extended the study of the gradients
of reduction to animal and vegetal halves and to vegetalized and
animalized embryos. We shall summarize these important observations.
R. LALLIER
animalization. The conjoined studies of animalization and vegetalization
processes are indeed essential for the understanding of embryonic
determination and differentiation.
IV. The Gradients
4 A gradient is a system of order involving progressively increasing or
decreasing entities or intensities from one pole or point of a morphogenetic field to another' (Needham, 1950). The interpretations of the
embryonic development of the sea urchin egg are dominated by the
notion of gradient. This notion is derived from morphological and
physiological observations.
According to Boveri (1901) the most vegetative parts exert a determinant control on the other parts of the egg. This control is maximal at
the vegetal pole and it extends, its influence decreasing, towards the
animal pole.
According to Child (1916, 1928, 1936, 1941) a single physiological
gradient controls morphogenesis. This gradient shows a quantitative
decrease along the axis of the organism, the maximum value being
situated at the place where morphogenesis is most active. Child demonstrated the existence of this physiological gradient by diverse processes
(differential susceptibility to toxic agents, reduction during anaerobiosis
of oxidation-reduction indicators).
Runnström (1928a, b, 1929) suggested that there exist two antagonistic
metabolic gradients qualitatively different and overlapping one another.
One of these gradients has its maximum at the animal pole, the other at
the vegetal pole. The interaction between these two gradients would
control the differentiation of the embryo.
Gradients of reduction are observed during the reduction of methylene
blue, or Janus green, by embryos in anaerobiosis. These gradients
indicate a differential distribution of metabolic activities along the
embryonic axis.
Child (1936) has noted during the segmentation of the sea urchin egg,
and in the young blastula, a gradient of reduction decreasing from the
animal pole to the vegetal pole (basipetal gradient). In the advanced
blastula, and during the gastrulation, a second gradient of reduction
appears, directed from the vegetal pole to the animal pole (acropetal
gradient). This acropetal gradient is stronger than the basipetal one and
coexists with it.
Hörstadius (1952, 1955) has confirmed these observations in normal
eggs (Fig. 3). Further, Hörstadius has extended the study of the gradients
of reduction to animal and vegetal halves and to vegetalized and
animalized embryos. We shall summarize these important observations.