380
JEAN J. PASTEELS
tion of the embryonic axis, takes place before the 8-cell stage; but his
interpretation of the active mechanism differs fundamentally from ours.
The theory of an interaction between cortical field and inner yolk
gradient (Dalcq and Pasteels, 1937, 1938; Dalcq, 1941; Pasteels,
1953) has been based on Pasteels' inversion and centrifugation experiments (1938b, 1939, 1940a, b, 1941, 1946, 1948) whose main points
have been summarized above ; this implies, of course, some spreading
of morphogenetic factors into the inner material. This conclusion
may have been put in question by the experiments of Curtis, who
obtained a negative result from grafting subcortical material, underlying the grey crescent, taken at the 8-cell stage. But we have serious
doubts about the methodological value of such an experiment. A
negative result has only some meaning when opposed to a positive one
obtained by the same technique. We may question the integrity of a
slice of subcortical material which is exposed on both faces by a crude
section before grafting. Curtis' interpretation is the following: in the
unsegmented egg, the grey-crescent cortex does not act as the focus of a
field (still non-existent at this stage), but as a 'centre for activation'. At
some time between the first and the third cleavage, this centre would
activate the establishment of a cortical field.
This 'centre of activation', unless it acts at some distance — i.e. as a
field — cannot give any explanation of the occurrence of gastrulation
( + embryonic axis) at the animal or vegetal pole (see above) and is
directly contradicted by the experiments of Dollander (1950) in which a
normal embryo is obtained from the ventral blastomere isolated at the
2-cell stage.
VI. Separation of Blastomeres by Ligature
Herlitzka (1896) developed the standard technique to separate the
two first blastomeres of Triton eggs by progressive constriction. He was
the first to show that a complete embryo may be obtained from both
blastomeres, although in other cases one of the partners was unable to
develop more than incipient gastrulation. Later Spemann (1901, 1902)
came to the conclusion that in the case of a coincidence between the
planes of first cleavage and of bilateral symmetry, complete embryos
were obtained from isolated blastomeres. On the other hand, when
ligature happened to separate a dorsal from a ventral blastomere, a
whole embryo was derived from the dorsal cell, whereas only a tentative
gastrula was given by the ventral one ('Bauchstück'). But in fact
Spemann, either in this work, or in further works on the same subject
(Spemann and Falkenberg, 1919; Ruud and Spemann, 1922), did not
recognize the orientation of the embryo before the experiment and the
JEAN J. PASTEELS
tion of the embryonic axis, takes place before the 8-cell stage; but his
interpretation of the active mechanism differs fundamentally from ours.
The theory of an interaction between cortical field and inner yolk
gradient (Dalcq and Pasteels, 1937, 1938; Dalcq, 1941; Pasteels,
1953) has been based on Pasteels' inversion and centrifugation experiments (1938b, 1939, 1940a, b, 1941, 1946, 1948) whose main points
have been summarized above ; this implies, of course, some spreading
of morphogenetic factors into the inner material. This conclusion
may have been put in question by the experiments of Curtis, who
obtained a negative result from grafting subcortical material, underlying the grey crescent, taken at the 8-cell stage. But we have serious
doubts about the methodological value of such an experiment. A
negative result has only some meaning when opposed to a positive one
obtained by the same technique. We may question the integrity of a
slice of subcortical material which is exposed on both faces by a crude
section before grafting. Curtis' interpretation is the following: in the
unsegmented egg, the grey-crescent cortex does not act as the focus of a
field (still non-existent at this stage), but as a 'centre for activation'. At
some time between the first and the third cleavage, this centre would
activate the establishment of a cortical field.
This 'centre of activation', unless it acts at some distance — i.e. as a
field — cannot give any explanation of the occurrence of gastrulation
( + embryonic axis) at the animal or vegetal pole (see above) and is
directly contradicted by the experiments of Dollander (1950) in which a
normal embryo is obtained from the ventral blastomere isolated at the
2-cell stage.
VI. Separation of Blastomeres by Ligature
Herlitzka (1896) developed the standard technique to separate the
two first blastomeres of Triton eggs by progressive constriction. He was
the first to show that a complete embryo may be obtained from both
blastomeres, although in other cases one of the partners was unable to
develop more than incipient gastrulation. Later Spemann (1901, 1902)
came to the conclusion that in the case of a coincidence between the
planes of first cleavage and of bilateral symmetry, complete embryos
were obtained from isolated blastomeres. On the other hand, when
ligature happened to separate a dorsal from a ventral blastomere, a
whole embryo was derived from the dorsal cell, whereas only a tentative
gastrula was given by the ventral one ('Bauchstück'). But in fact
Spemann, either in this work, or in further works on the same subject
(Spemann and Falkenberg, 1919; Ruud and Spemann, 1922), did not
recognize the orientation of the embryo before the experiment and the