CELL DIVISIONS
357
to its prospective significance (Holtfreter, 1925), and the entodermal
material from the neurula is only capable of a morphological regulation
(Kemp, 1946), the latent differentiation of entodermal rudiments in the
urodeles remains labile even at the end of gastrulation and neurulation
(Balinsky, 1939, 1948). This latter conclusion is supported by the new
data obtained by Okada (1957, 1960a, b) and Takata (1960a, b) on
Triturus
pyrrhogaster.
Comparing these data with those on the differences in the time of
intrinsic development of ectodermal derivatives, Ten Cate (1956) drew
attention to their similarity. He wrote that there were differences
between the anurans and the urodeles in general which were most
demonstratively revealed in the determination of the sense and
ectodermal organs.
With respect to the times of latent differentiation of mesodermal
derivatives, of particular interest are the results of Pasteels' (1953a, b,
1954) experiments on centrifugation of the embryos of various amphibian
species at the stages from early blastula up to the early gastrula. Under
the influence of centrifugation the ectoblast acquires the ability to
undergo differentiation and to form a complex of neuromesodermal
structures. It appeared that such a response occurred in different animal
species at different developmental stages. It appears in Xenopus
laevis
earliest of all followed by Rana temporaria. The ectoblast in Ambystoma
and Pleurodeles is of a lower reactivity than that in Xenopus and Rana.
The period of sensitivity in Ambystoma punctatum falls in an earlier
stage than that of Pleurodeles.
The sensitivity of the ectoblast in
Discoglossus is low, while three newt species (Triturus vulgaris,
T.
helveticus and T. alpestris) lack sensitivity to centrifugation from the
early blastula to the early gastrula. When centrifuged they form
no neuromesodermal structures.
The differences in the rate of differentiation of the ectoderm and of
the chorda-mesodermal rudiment are revealed not only with respect to
their formative properties but also by time of determination of polarity
(Luther, 1934; Dettlaff, 1947) and of their segregation into layers
(Lehmann, 1938 ; Dettlaff, 1958). These processes in the anurans occur at
earlier stages than in the urodeles.
On the basis of all the data presented Dettlaff (1956) has concluded
that 'species specific differences exist in the rate of the differentiation of
the material of different germinal layers and their derivatives with
respect to general developmental processes (gastrulation, neurulation).
These differences lead to a change in the relative importance of individual
formative interactions/ As a matter of fact, differences in the character
of formative interactions in the anurans, urodeles and sturgeon fishes
correspond to the different differentiation tempo of their ectoderm and
357
to its prospective significance (Holtfreter, 1925), and the entodermal
material from the neurula is only capable of a morphological regulation
(Kemp, 1946), the latent differentiation of entodermal rudiments in the
urodeles remains labile even at the end of gastrulation and neurulation
(Balinsky, 1939, 1948). This latter conclusion is supported by the new
data obtained by Okada (1957, 1960a, b) and Takata (1960a, b) on
Triturus
pyrrhogaster.
Comparing these data with those on the differences in the time of
intrinsic development of ectodermal derivatives, Ten Cate (1956) drew
attention to their similarity. He wrote that there were differences
between the anurans and the urodeles in general which were most
demonstratively revealed in the determination of the sense and
ectodermal organs.
With respect to the times of latent differentiation of mesodermal
derivatives, of particular interest are the results of Pasteels' (1953a, b,
1954) experiments on centrifugation of the embryos of various amphibian
species at the stages from early blastula up to the early gastrula. Under
the influence of centrifugation the ectoblast acquires the ability to
undergo differentiation and to form a complex of neuromesodermal
structures. It appeared that such a response occurred in different animal
species at different developmental stages. It appears in Xenopus
laevis
earliest of all followed by Rana temporaria. The ectoblast in Ambystoma
and Pleurodeles is of a lower reactivity than that in Xenopus and Rana.
The period of sensitivity in Ambystoma punctatum falls in an earlier
stage than that of Pleurodeles.
The sensitivity of the ectoblast in
Discoglossus is low, while three newt species (Triturus vulgaris,
T.
helveticus and T. alpestris) lack sensitivity to centrifugation from the
early blastula to the early gastrula. When centrifuged they form
no neuromesodermal structures.
The differences in the rate of differentiation of the ectoderm and of
the chorda-mesodermal rudiment are revealed not only with respect to
their formative properties but also by time of determination of polarity
(Luther, 1934; Dettlaff, 1947) and of their segregation into layers
(Lehmann, 1938 ; Dettlaff, 1958). These processes in the anurans occur at
earlier stages than in the urodeles.
On the basis of all the data presented Dettlaff (1956) has concluded
that 'species specific differences exist in the rate of the differentiation of
the material of different germinal layers and their derivatives with
respect to general developmental processes (gastrulation, neurulation).
These differences lead to a change in the relative importance of individual
formative interactions/ As a matter of fact, differences in the character
of formative interactions in the anurans, urodeles and sturgeon fishes
correspond to the different differentiation tempo of their ectoderm and