14
JANE Μ. OPPENHEIMER
related to the activities of specific metabolic systems, respiratory and
otherwise (Hörstadius, 1928, 1935, 1939, 1949; Runnström, 1928, 1954;
Lindahl, 1936, 1942; Gustaf son and Lenique, 1952, 1955). In fact, if this
essay were being written by an echinoderm rather than a vertebrate, the
establishment and elaboration of gradients would almost certainly be
considered to be the most highly significant of the organizing mechanisms in development that relate a governed part to a governing whole.
Yet even the echinoderms might be driven to the admission that the
concepts of gradients and fields describe principally differences in action
and reaction between parts of an organism at a specified moment in
time; and dynamic though they may be in other respects, they have perhaps their greatest value in the delineation of spatial rather than temporal situations. Something further is requisite to account for changes in
gradients and fields in time. The concepts implicit in organizer action,
based as they are on mechanisms of induction, deal to a far greater degree with processes responsible for the progression of events through
time; and in fact it is when these are superimposed on gradient concepts,
as for instance when von Ubisch (1925) and Hörstadius (1928, 1935)
alter the later development of echinoderm embryonic cells at one end
of a gradient by the addition or subtraction of cells originating from elsewhere along the gradient, that they achieve their most spectacular
success. Since differentiation is a process in time rather than a fixed
state (a fact which many contemporary biologists tend to neglect),
the ultimate task of embryology and developmental physiology becomes
the analysis of sequences of events in time, and theories which account
for spatial distinctions between parts of embryos, no matter how profound these may be, must always remain subservient to hypotheses which
emphasize the progressive element in differentiation. The experiments
on which the organizer concept was based have made manifest the degree
to which phenomena and events of any one particular moment in development are dynamically related to events that have occurred at
previous moments. Further, they have provided incontrovertible proof
that a mutual influence between embryonic parts lies at the basis of the
complex interrelationships of embryonic processes. Thus they have
imputed the ultimate validity and significance to the fundamental principle of progressive differentiation which has dominated all of the
concepts of embryology since the time of Aristotle.
Considerable space has been devoted in the immediately foregoing
pages to the origins and consequences, to the likenesses and similarities,
of the notions of organizers and gradients and fields, not only because of
the intrinsic interest of these ideas but also because they have been ex-
JANE Μ. OPPENHEIMER
related to the activities of specific metabolic systems, respiratory and
otherwise (Hörstadius, 1928, 1935, 1939, 1949; Runnström, 1928, 1954;
Lindahl, 1936, 1942; Gustaf son and Lenique, 1952, 1955). In fact, if this
essay were being written by an echinoderm rather than a vertebrate, the
establishment and elaboration of gradients would almost certainly be
considered to be the most highly significant of the organizing mechanisms in development that relate a governed part to a governing whole.
Yet even the echinoderms might be driven to the admission that the
concepts of gradients and fields describe principally differences in action
and reaction between parts of an organism at a specified moment in
time; and dynamic though they may be in other respects, they have perhaps their greatest value in the delineation of spatial rather than temporal situations. Something further is requisite to account for changes in
gradients and fields in time. The concepts implicit in organizer action,
based as they are on mechanisms of induction, deal to a far greater degree with processes responsible for the progression of events through
time; and in fact it is when these are superimposed on gradient concepts,
as for instance when von Ubisch (1925) and Hörstadius (1928, 1935)
alter the later development of echinoderm embryonic cells at one end
of a gradient by the addition or subtraction of cells originating from elsewhere along the gradient, that they achieve their most spectacular
success. Since differentiation is a process in time rather than a fixed
state (a fact which many contemporary biologists tend to neglect),
the ultimate task of embryology and developmental physiology becomes
the analysis of sequences of events in time, and theories which account
for spatial distinctions between parts of embryos, no matter how profound these may be, must always remain subservient to hypotheses which
emphasize the progressive element in differentiation. The experiments
on which the organizer concept was based have made manifest the degree
to which phenomena and events of any one particular moment in development are dynamically related to events that have occurred at
previous moments. Further, they have provided incontrovertible proof
that a mutual influence between embryonic parts lies at the basis of the
complex interrelationships of embryonic processes. Thus they have
imputed the ultimate validity and significance to the fundamental principle of progressive differentiation which has dominated all of the
concepts of embryology since the time of Aristotle.
Considerable space has been devoted in the immediately foregoing
pages to the origins and consequences, to the likenesses and similarities,
of the notions of organizers and gradients and fields, not only because of
the intrinsic interest of these ideas but also because they have been ex-