TWENTIETH CENTURY EMBRYOLOGICAL CONCEPTS
3
einer grösseren Bedeutung gelangt, wenn dieselben nicht in v. BAER
einen Förderer und theilweise auch einen Vertreter gefunden
hätten, der es verstand, der Blättertheorie in den weitesten Kreisen
Eingang zu verschaffen."
3
Haeckel, in contrast, fifty years after his moment in history was certainly evaluated quite differently than we interpret him today: Kerr in
a brief history of the germ layer theory published in 1919 could write
(p. 506) of the Gastraea theory quite factually and uncritically that:
"Haeckel. . . about the same time as Lankester also developed the
idea that the diploblastic stage of ontogeny was to be interpreted as
the repetition of an ancestral form: Haeckel called this ancestral
form Gastraea.
The main difference between Haeckel's view and
Lankester s was that the former regarded the endoderm as having
arisen by a process of invagination—as it actually does arise in ontogeny in the great majority of cases—while Lankester regarded it
as having arisen by a process of delamination from the outer layer."
He terminated his historical discussion of the establishment of the germ
layer theory (published, it may be noticed, the year after Spemann's
first exhaustive communication on the relationships of the layers during
gastrulation) with only a single and rather insignificant reservation:
"The author regards as the chief qualification of the germ layer theory
indicated by modern work . . . that the boundary between two layers
where they are continued into one another must be regarded not as a
sharply marked line but as a more or less broad debatable zone" (Kerr,
1919). It is an interesting and perhaps meaningful fact that the view
expressed a half century later concerning the contribution of Pander, who
flourished early in the 19th century, has changed less during the ensuing
years that the evaluation, after an approximate half century, of Haeckel
who worked closer to our own time; clearly the passage of time as
3 Pander's investigations not only present a more exact history of the earliest development of the chick than was previously available; but they were of the greatest
importance since it was through these studies that the primitive organs, foreseen by
Wolff, which lie at the basis of the development of later organs and tissues, were
for the first time authenticated by observation. Pander distinguished two layers in
the blastoderm of the chick, an outer . . . and an inner . . . between which a third
layer . . . later develops. Although Pander considered these layers as the point of
origin of all later organs, yet he expressed himself only very briefly concerning their
transformations and general significance; and because of the aphoristic manner of
his presentation his statements would not so soon have attained their great significance had they not found in von Baer a promoter and partly an advocate who understood how to bring the layer theory into fashion in the widest circles.
3
einer grösseren Bedeutung gelangt, wenn dieselben nicht in v. BAER
einen Förderer und theilweise auch einen Vertreter gefunden
hätten, der es verstand, der Blättertheorie in den weitesten Kreisen
Eingang zu verschaffen."
3
Haeckel, in contrast, fifty years after his moment in history was certainly evaluated quite differently than we interpret him today: Kerr in
a brief history of the germ layer theory published in 1919 could write
(p. 506) of the Gastraea theory quite factually and uncritically that:
"Haeckel. . . about the same time as Lankester also developed the
idea that the diploblastic stage of ontogeny was to be interpreted as
the repetition of an ancestral form: Haeckel called this ancestral
form Gastraea.
The main difference between Haeckel's view and
Lankester s was that the former regarded the endoderm as having
arisen by a process of invagination—as it actually does arise in ontogeny in the great majority of cases—while Lankester regarded it
as having arisen by a process of delamination from the outer layer."
He terminated his historical discussion of the establishment of the germ
layer theory (published, it may be noticed, the year after Spemann's
first exhaustive communication on the relationships of the layers during
gastrulation) with only a single and rather insignificant reservation:
"The author regards as the chief qualification of the germ layer theory
indicated by modern work . . . that the boundary between two layers
where they are continued into one another must be regarded not as a
sharply marked line but as a more or less broad debatable zone" (Kerr,
1919). It is an interesting and perhaps meaningful fact that the view
expressed a half century later concerning the contribution of Pander, who
flourished early in the 19th century, has changed less during the ensuing
years that the evaluation, after an approximate half century, of Haeckel
who worked closer to our own time; clearly the passage of time as
3 Pander's investigations not only present a more exact history of the earliest development of the chick than was previously available; but they were of the greatest
importance since it was through these studies that the primitive organs, foreseen by
Wolff, which lie at the basis of the development of later organs and tissues, were
for the first time authenticated by observation. Pander distinguished two layers in
the blastoderm of the chick, an outer . . . and an inner . . . between which a third
layer . . . later develops. Although Pander considered these layers as the point of
origin of all later organs, yet he expressed himself only very briefly concerning their
transformations and general significance; and because of the aphoristic manner of
his presentation his statements would not so soon have attained their great significance had they not found in von Baer a promoter and partly an advocate who understood how to bring the layer theory into fashion in the widest circles.