I. PROBLEM
OF THE
ORGANIZER
49
reflecting a change in the synthetic activity of the reacting cell whether
it is occurring in the cytoplasm or in the nucleus. The well known
intimate contact of the organizer layer with the ectoderm, not observable at other boundaries of the germ layers may also be indicative of the
specific synthetic activity of the organizer which has been suggested by
the immunological test of Vainio (1956).
Before closing this chapter, brief mention should be made of the
problem of embryonic organization. Although various ideas have been
proposed in the last decade concerning the regional organization of the
embryo, all of them have in common the tendency to avoid both
extreme preformation and extreme epigénesis. Yamada (1950b) emphasized two qualitatively different aspects of morphogenetic activity, which characterize dorso-ventral and cephalo-caudal axes of the
future embryo. The archencephalic induction is brought about by
the factor related to the dorso-ventrality, while deuterencephalic and
spino-caudal inductions require beside the same factor the second one
which is related to the cephalo-caudality. The level of the activity of
the second factor is higher in the spino-caudal induction than in the
deuterencephalic induction. According to Nieuwkoop et al. (1952), and
Nieuwkoop (1955), the inductive influence of the organizer can be
analysed into two activities: activation which occurs first, affects all
neural areas, and leads to archencephalic differentiation, and transformation which occurs later, affects only the posterior neural area, and
leads, in combination with activation, to deuterencephalic or spinocaudal differentiation. Toivonen and Saxén (1955b) proposed a theory of
regional induction in which a neural agent and mesodermal agent are
assumed. The neural agent alone causes archencephalic induction, while
combination of neural and mesodermal agents lead to deuterencephalic
or spino-caudal induction. On the other hand, Dalcq (1957b) suggested
the idea according to which the morphogenetic property of the organizer
cell changes its regional effect from spino-caudal to archencephalic
during the course of invagination. All these ideas admit a qualitative
difference in the inducing mechanism along the cephalo-caudal axis,
which can be reduced to two basic factors. The increasing body of
evidence in support of the progressive change of regional effects of the
inductor caused by divergent denaturating treatments (c/. Yamada,
1958c, 1959), and the combinative effects of two different types of
inductor (Toivonen and Saxén, 1955a, b) seems to promise an approach
to the problem from the physical property of the macromolecules
responsible for regional induction.
OF THE
ORGANIZER
49
reflecting a change in the synthetic activity of the reacting cell whether
it is occurring in the cytoplasm or in the nucleus. The well known
intimate contact of the organizer layer with the ectoderm, not observable at other boundaries of the germ layers may also be indicative of the
specific synthetic activity of the organizer which has been suggested by
the immunological test of Vainio (1956).
Before closing this chapter, brief mention should be made of the
problem of embryonic organization. Although various ideas have been
proposed in the last decade concerning the regional organization of the
embryo, all of them have in common the tendency to avoid both
extreme preformation and extreme epigénesis. Yamada (1950b) emphasized two qualitatively different aspects of morphogenetic activity, which characterize dorso-ventral and cephalo-caudal axes of the
future embryo. The archencephalic induction is brought about by
the factor related to the dorso-ventrality, while deuterencephalic and
spino-caudal inductions require beside the same factor the second one
which is related to the cephalo-caudality. The level of the activity of
the second factor is higher in the spino-caudal induction than in the
deuterencephalic induction. According to Nieuwkoop et al. (1952), and
Nieuwkoop (1955), the inductive influence of the organizer can be
analysed into two activities: activation which occurs first, affects all
neural areas, and leads to archencephalic differentiation, and transformation which occurs later, affects only the posterior neural area, and
leads, in combination with activation, to deuterencephalic or spinocaudal differentiation. Toivonen and Saxén (1955b) proposed a theory of
regional induction in which a neural agent and mesodermal agent are
assumed. The neural agent alone causes archencephalic induction, while
combination of neural and mesodermal agents lead to deuterencephalic
or spino-caudal induction. On the other hand, Dalcq (1957b) suggested
the idea according to which the morphogenetic property of the organizer
cell changes its regional effect from spino-caudal to archencephalic
during the course of invagination. All these ideas admit a qualitative
difference in the inducing mechanism along the cephalo-caudal axis,
which can be reduced to two basic factors. The increasing body of
evidence in support of the progressive change of regional effects of the
inductor caused by divergent denaturating treatments (c/. Yamada,
1958c, 1959), and the combinative effects of two different types of
inductor (Toivonen and Saxén, 1955a, b) seems to promise an approach
to the problem from the physical property of the macromolecules
responsible for regional induction.