556
E T I E N N E W O L F F
thought that Hensen's node and the anterior edges of the primitive
streak, homologues of the dorsal blastoporal lip, must play an organizing role in the early development of birds.
Waddington demonstrated this (1932, 1934) by using the in vitro
culture technique. At the primitive streak stage he detached the
mesectoderm from the endoderm. The mesectoderm of the embryo was
placed with its ventral surface against the ventral surface of another
mesectoderm. It was found that if the primitive streak of the bottom
layer was placed against the extra-embryonic area of the top mesectoderm, it induced the formation of a neural plate in the ectoderm.
Anterior fragments of the primitive streak grafted under the ectoderm
have inducing properties, but posterior fragments do not. They appear
to be particularly active in the vicinity of Hensen's node. This result
tends to show that the chordo-somitic mesoderm, and not the lateral
one, is endowed with inducing properties.
The anterior part of the primitive streak maintains its activity after
coagulation by heat. The analogy with amphibians is further strengthened by the fact that the inducer has no species-specificity: Waddington
and Schmidt (1933) obtained neural inductions in duck with a primitive
streak from chick; and, inversely, neural plates developed in chick
with an inducer from duck. Further, the primitive streak from chick
can induce neural formations in the blastoderm of mammals (Waddington, 1934, III). A very interesting property has been attributed to the
endodermal layer at the stage ofthe early primitive streak: it determines
the orientation of the primitive streak and of the organs in the axial
plane of the embryo. If, having first separated the ectomesoderm from
the endoderm the two layers are then replaced one on top of the other
but turned through 180°, it is often found that a second primitive streak
is formed and that therefore two embryonic axes, facing in opposite
directions, result. Sometimes, the initial primitive streak disappears
and the induced streak becomes the only axis of development.
When the two layers are turned through 90°, the first primitive
streak disappears and a new embryonic axis forms, facing in a direction
intermediate between the antero-posterior axes resulting from the two
successive positions of the endoderm. This shows not only that the
endodermal layer sets the direction of the embryonic axis, but shows also
that a certain territory of this layer, situated in what will be the posteromedian region, has a determining influence on the localization of the
primitive streak.
These investigations clearly demonstrate that a homology does exist
between the anterior part of the primitive streak and the dorsal blastoporal lip of amphibians. Grabowski (1957) has added further details
to the results of Waddington's work. He has shown that the region of
E T I E N N E W O L F F
thought that Hensen's node and the anterior edges of the primitive
streak, homologues of the dorsal blastoporal lip, must play an organizing role in the early development of birds.
Waddington demonstrated this (1932, 1934) by using the in vitro
culture technique. At the primitive streak stage he detached the
mesectoderm from the endoderm. The mesectoderm of the embryo was
placed with its ventral surface against the ventral surface of another
mesectoderm. It was found that if the primitive streak of the bottom
layer was placed against the extra-embryonic area of the top mesectoderm, it induced the formation of a neural plate in the ectoderm.
Anterior fragments of the primitive streak grafted under the ectoderm
have inducing properties, but posterior fragments do not. They appear
to be particularly active in the vicinity of Hensen's node. This result
tends to show that the chordo-somitic mesoderm, and not the lateral
one, is endowed with inducing properties.
The anterior part of the primitive streak maintains its activity after
coagulation by heat. The analogy with amphibians is further strengthened by the fact that the inducer has no species-specificity: Waddington
and Schmidt (1933) obtained neural inductions in duck with a primitive
streak from chick; and, inversely, neural plates developed in chick
with an inducer from duck. Further, the primitive streak from chick
can induce neural formations in the blastoderm of mammals (Waddington, 1934, III). A very interesting property has been attributed to the
endodermal layer at the stage ofthe early primitive streak: it determines
the orientation of the primitive streak and of the organs in the axial
plane of the embryo. If, having first separated the ectomesoderm from
the endoderm the two layers are then replaced one on top of the other
but turned through 180°, it is often found that a second primitive streak
is formed and that therefore two embryonic axes, facing in opposite
directions, result. Sometimes, the initial primitive streak disappears
and the induced streak becomes the only axis of development.
When the two layers are turned through 90°, the first primitive
streak disappears and a new embryonic axis forms, facing in a direction
intermediate between the antero-posterior axes resulting from the two
successive positions of the endoderm. This shows not only that the
endodermal layer sets the direction of the embryonic axis, but shows also
that a certain territory of this layer, situated in what will be the posteromedian region, has a determining influence on the localization of the
primitive streak.
These investigations clearly demonstrate that a homology does exist
between the anterior part of the primitive streak and the dorsal blastoporal lip of amphibians. Grabowski (1957) has added further details
to the results of Waddington's work. He has shown that the region of