15. E M B R Y O G E N E S I S in vitro
557
Hensen's node, taken from an embryo and implanted between the
ectoderm and the endoderm of another blastoderm and outside the
axial area of the embryo, induces the formation of a secondary embryo.
The inducing function of Hensen's node and of different parts of the
head process was analysed very recently by Hara (1961). This author
used a method which combined explantation and coelomic grafts. He
prepared fragments of antero-lateral ectoderm which had been carefully
separated from the mesoderm (something which was not achieved by
previous authors). He put them in culture in association with anterior,
middle and posterior parts of the head process (chordal and prechordal
rudiments): thus "open sandwiches" were prepared and they were maintained in culture until the two tissues were welded together. U p to this
point, this method is very similar to that of "ectodermal sandwiches"
used with amphibians. However, since it did not allow the explants to
become differentiated sufficiently long to allow recognition of the regional nature of the neural formation, Hara re-implanted his "sandwiches"
into a host, following the technique of coelomic grafting. One criticism
of this method is that, in the living embryo, the grafts are not free of the
influence of the hormonal milieu or of neighbouring tissues. Nevertheless, when the graft is judiciously controlled, such an objection does
not appear relevant. In general, prechordal mesoderm induces archencephalic formations and the anterior part of the head process induces
differentiations of mesencephalic and rhombencephalic nature (deuterencephalon) whereas the posterior part of the head process induces
only rhombencephalic and spinal formations (Fig. 13). Thus, the different regions of the nervous system are induced by the different parts
of the organizing centre, graded from front to back. An interesting
parallel between regional inductions in amphibians and birds is thus
evident.
2. Existence and Properties of Mesodermal Differentiation Centres in Birds
Spratt (1955, 1957a, b ) , starting with undifferentiated rudiments,
has studied the conditions in which the region of the mesodermal
organization centre becomes differentiated. His work was done in vitro
by means of localized lesions, sections of blastoderm, and explantations
of small areas from the vicinity of the primitive node.
The principal results of these researches can be summarized as follows:
The anterior region of the primitive streak marks the border between
differentiated territories and regions of presumptive chord and somites.
The question arises, therefore, whether these rudiments are determined,
or whether they are also subject to an induction which would cause
their differentiation into somites. Spratt showed that a chordal organizing centre does exist at the level of the primitive node. This centre is
557
Hensen's node, taken from an embryo and implanted between the
ectoderm and the endoderm of another blastoderm and outside the
axial area of the embryo, induces the formation of a secondary embryo.
The inducing function of Hensen's node and of different parts of the
head process was analysed very recently by Hara (1961). This author
used a method which combined explantation and coelomic grafts. He
prepared fragments of antero-lateral ectoderm which had been carefully
separated from the mesoderm (something which was not achieved by
previous authors). He put them in culture in association with anterior,
middle and posterior parts of the head process (chordal and prechordal
rudiments): thus "open sandwiches" were prepared and they were maintained in culture until the two tissues were welded together. U p to this
point, this method is very similar to that of "ectodermal sandwiches"
used with amphibians. However, since it did not allow the explants to
become differentiated sufficiently long to allow recognition of the regional nature of the neural formation, Hara re-implanted his "sandwiches"
into a host, following the technique of coelomic grafting. One criticism
of this method is that, in the living embryo, the grafts are not free of the
influence of the hormonal milieu or of neighbouring tissues. Nevertheless, when the graft is judiciously controlled, such an objection does
not appear relevant. In general, prechordal mesoderm induces archencephalic formations and the anterior part of the head process induces
differentiations of mesencephalic and rhombencephalic nature (deuterencephalon) whereas the posterior part of the head process induces
only rhombencephalic and spinal formations (Fig. 13). Thus, the different regions of the nervous system are induced by the different parts
of the organizing centre, graded from front to back. An interesting
parallel between regional inductions in amphibians and birds is thus
evident.
2. Existence and Properties of Mesodermal Differentiation Centres in Birds
Spratt (1955, 1957a, b ) , starting with undifferentiated rudiments,
has studied the conditions in which the region of the mesodermal
organization centre becomes differentiated. His work was done in vitro
by means of localized lesions, sections of blastoderm, and explantations
of small areas from the vicinity of the primitive node.
The principal results of these researches can be summarized as follows:
The anterior region of the primitive streak marks the border between
differentiated territories and regions of presumptive chord and somites.
The question arises, therefore, whether these rudiments are determined,
or whether they are also subject to an induction which would cause
their differentiation into somites. Spratt showed that a chordal organizing centre does exist at the level of the primitive node. This centre is