TWENTIETH CENTURY EMBRYOLOGICAL CONCEPTS
19
cal means.) The indication that even within each germ layer prospective
potency, to borrow for a moment some old-fashioned but useful terminology, may be greater than prospective significance, and that the increment varies for different organisms and for different layers in the same
organism, has resulted on the negative side in the weakening of the firm
ideas of germ layer fixity and specificity as a generalizing principle; and
on the positive, it has pointed up the need for formulating new interpretations of regulation and regulability, and thus of epigénesis and preformation. It was in part the breaking of the absolute rule of the old
germ layer dogma that released the energies of embryologists to treat
morphogenesis in the wide variety of manners which has been described
as characteristic of the early 20th century.
But if the degree of specificity formerly attributed to the layers now
seems devaluated, and if the present assessments of the layers are now
so changed that, for instance, von Baer and Haeckel might have difficulty in recognizing their legacy in the chemical coinage of Toivonen
and Yamada, nevertheless an intimate continuity has obtained between
the rigorous concept developed during the 19th century and some of the
more modern investigations carried out during the 20th. Vogt's magnificent studies on morphogenetic movements considered the rearrangements of cells in groups, and except for the chorda his groups were principally the old germ layers. Thus in a way he substituted dynamic and
material reality for what had been previously formal. Spemann analyzed
the operations of the organizer in terms of the action of an underlying
layer of cells on an overlying one. The fact that chorda could be demonstrated by modern methods as having position and function in its own
right, as distinct from both mesoderm and endoderm, would have given
small comfort to the adherents of the older and stricter theory. Yet the
embryo itself has proved that prospective notochord adapts its migrations to those of prospective mesoderm and that it cooperates with this
layer, at least in the urodele, in affecting the actions of the overlying
ectoderm.
But Spemann and his immediate collaborators were not the last to
utilize distinctions between the actions and reactions of the conventional
layers as keys by which to investigate induction. When Dalcq (Dalcq
and Pasteéis, 1937) explains qualitative diversity along the amphibian
embryonic axis by postulating quantitative distribution of a hypothetical
substance (organisine) along a hypothetical morphological gradient in
the mesoderm, he evinces his debt for the only certainty in his explanation to the germ layer concept which has defined for him the existence
of mesoderm as a distinct entity. The same holds true for Nieuwkoop
19
cal means.) The indication that even within each germ layer prospective
potency, to borrow for a moment some old-fashioned but useful terminology, may be greater than prospective significance, and that the increment varies for different organisms and for different layers in the same
organism, has resulted on the negative side in the weakening of the firm
ideas of germ layer fixity and specificity as a generalizing principle; and
on the positive, it has pointed up the need for formulating new interpretations of regulation and regulability, and thus of epigénesis and preformation. It was in part the breaking of the absolute rule of the old
germ layer dogma that released the energies of embryologists to treat
morphogenesis in the wide variety of manners which has been described
as characteristic of the early 20th century.
But if the degree of specificity formerly attributed to the layers now
seems devaluated, and if the present assessments of the layers are now
so changed that, for instance, von Baer and Haeckel might have difficulty in recognizing their legacy in the chemical coinage of Toivonen
and Yamada, nevertheless an intimate continuity has obtained between
the rigorous concept developed during the 19th century and some of the
more modern investigations carried out during the 20th. Vogt's magnificent studies on morphogenetic movements considered the rearrangements of cells in groups, and except for the chorda his groups were principally the old germ layers. Thus in a way he substituted dynamic and
material reality for what had been previously formal. Spemann analyzed
the operations of the organizer in terms of the action of an underlying
layer of cells on an overlying one. The fact that chorda could be demonstrated by modern methods as having position and function in its own
right, as distinct from both mesoderm and endoderm, would have given
small comfort to the adherents of the older and stricter theory. Yet the
embryo itself has proved that prospective notochord adapts its migrations to those of prospective mesoderm and that it cooperates with this
layer, at least in the urodele, in affecting the actions of the overlying
ectoderm.
But Spemann and his immediate collaborators were not the last to
utilize distinctions between the actions and reactions of the conventional
layers as keys by which to investigate induction. When Dalcq (Dalcq
and Pasteéis, 1937) explains qualitative diversity along the amphibian
embryonic axis by postulating quantitative distribution of a hypothetical
substance (organisine) along a hypothetical morphological gradient in
the mesoderm, he evinces his debt for the only certainty in his explanation to the germ layer concept which has defined for him the existence
of mesoderm as a distinct entity. The same holds true for Nieuwkoop