350
T. A. DETTLAFF
transition from a flexible state of pluripotency to a fixed state takes
place under the conditions of explantation. In this fixed state the
direction of further development of the parts is strictly determined.
It should be added that this process in the expiants is slower than in an
intact embryo. In the experiments carried out by Okada, as well as in
those of Holtfreter (1938a) and Gallera (1952), the properties of the
explanted rudiments were affected by the length of their stay in the
saline solution, hence the total duration of the interkinetic state of the
cells of the explanted rudiments is of morphogenetic importance.
D. Importance of the Duration of the Interkinetic State and Number of
Cell Divisions for the Process of Latent Differentiation of the Entoderm
The amphibian entoderm is formed from vegetative blastomeres
which are characterized not only by larger size and more yolk but also
by their mitotic index which during asynchronous divisions and
gastrulation is considerably lower than that of the animal blastomeres.
The mitotic index at the blastula stage of Bufo cognatus (Bragg, 1938) is
64-8 in animal blastomeres and 44-5 in large vegetative ones; at the
stage of the blastopore it is 81· 1 at the apex of the animal region
(presumptive ectoderm), 58-4 in the blastopore region (presumptive
chorda-mesoderm?) and only 12-7 in the macromeres (presumptive
entoderm). During gastrulation the mitotic index drops sharply (the
average is 7-7) being relatively smaller in entodermal cells (5-9) than in
other parts of the embryo (8-8). Thus entodermal cells possess a greater
sum total duration of the interkinetic state at the same developmental
stages than do ectodermal cells. In conformity with this the latent
differentiation of anuran entoderm can be revealed in experimental
conditions earlier than that of the ectoderm (Holtfreter, 1938b).
In this connection the experimental results obtained by Takata and
Yamada (1960) are of a great interest. They transplanted guinea pig
bone marrow previously treated with ethanol between two ectodermal
layers of the early gastrula in Triturus pyrrhogaster. After 10-14 days of
cultivation the expiants contained, along with mesodermal structures,
entodermal ones (pharynx, intestine, etc). Takata and Yamada suggested
that the induction of the entoderm from the ectoderm was produced by
a delay in the divisions of the ectodermal cells. Unfortunately, this
important conclusion is supported only by the fact that the entodermal
cells in the expiants are larger than the mesodermal ones ; but the size of
the cells after 10 to 14 days of cultivation could be determined not only
by the number of cell divisions but also by the specificity of their growth.
If the conclusion could be demonstrated by observations on younger
expiants, this would be direct evidence that, besides the level of
ooplasmic segregation, the combination of cell generations and the
T. A. DETTLAFF
transition from a flexible state of pluripotency to a fixed state takes
place under the conditions of explantation. In this fixed state the
direction of further development of the parts is strictly determined.
It should be added that this process in the expiants is slower than in an
intact embryo. In the experiments carried out by Okada, as well as in
those of Holtfreter (1938a) and Gallera (1952), the properties of the
explanted rudiments were affected by the length of their stay in the
saline solution, hence the total duration of the interkinetic state of the
cells of the explanted rudiments is of morphogenetic importance.
D. Importance of the Duration of the Interkinetic State and Number of
Cell Divisions for the Process of Latent Differentiation of the Entoderm
The amphibian entoderm is formed from vegetative blastomeres
which are characterized not only by larger size and more yolk but also
by their mitotic index which during asynchronous divisions and
gastrulation is considerably lower than that of the animal blastomeres.
The mitotic index at the blastula stage of Bufo cognatus (Bragg, 1938) is
64-8 in animal blastomeres and 44-5 in large vegetative ones; at the
stage of the blastopore it is 81· 1 at the apex of the animal region
(presumptive ectoderm), 58-4 in the blastopore region (presumptive
chorda-mesoderm?) and only 12-7 in the macromeres (presumptive
entoderm). During gastrulation the mitotic index drops sharply (the
average is 7-7) being relatively smaller in entodermal cells (5-9) than in
other parts of the embryo (8-8). Thus entodermal cells possess a greater
sum total duration of the interkinetic state at the same developmental
stages than do ectodermal cells. In conformity with this the latent
differentiation of anuran entoderm can be revealed in experimental
conditions earlier than that of the ectoderm (Holtfreter, 1938b).
In this connection the experimental results obtained by Takata and
Yamada (1960) are of a great interest. They transplanted guinea pig
bone marrow previously treated with ethanol between two ectodermal
layers of the early gastrula in Triturus pyrrhogaster. After 10-14 days of
cultivation the expiants contained, along with mesodermal structures,
entodermal ones (pharynx, intestine, etc). Takata and Yamada suggested
that the induction of the entoderm from the ectoderm was produced by
a delay in the divisions of the ectodermal cells. Unfortunately, this
important conclusion is supported only by the fact that the entodermal
cells in the expiants are larger than the mesodermal ones ; but the size of
the cells after 10 to 14 days of cultivation could be determined not only
by the number of cell divisions but also by the specificity of their growth.
If the conclusion could be demonstrated by observations on younger
expiants, this would be direct evidence that, besides the level of
ooplasmic segregation, the combination of cell generations and the