CELL DIVISIONS
329
of adjacent normal blastomeres, while enucleated animal blastomeres
show this effect slightly, or not at all (Hörstadius et al., 1953).
A diminished competence of tissues is also found in the lethal hybrid
Rana pipiens $ χ Rana sylvatica $ (J. Moore, 1947, 1948). The ectoblast
of the early gastrula of hybrids, when transplanted to the flank of a
neurula of Rana palustris or Ambystoma
maculatum, forms nervous
tissue, sense organs, pronephros and other rudiments. Induced structures, however, appear in transplanted hybrid tissue less frequently,
they are smaller in size and less differentiated than the similar rudiments
from the ectoblast of control embryos of Rana pipiens. In a similar way,
the material of the dorsal blastopore lip of these hybrids, when transplanted to the blastocoele of the normal gastrula of Rana palustris, has a
much weaker inducing action than the dorsal blastopore lip of a normal
Rana pipiens embryo in the control. In this case secondary structures
also appear in fewer cases, they are smaller in size and more weakly
differentiated. Moore (1947) writes in this connection: 'Thus the effects
of hybridization are not confined to the factors responsible for gastrulation alone but also involve those necessary for the attainment of full
competence'.
The case of the anomalous egg development of one sevruga female
studied by Skoblina (1964) can be interpreted in the same way. Mosaics
of normal and atypical cells appeared in some eggs of this female. After a
short exposure to high temperature at the stage of the first cleavage
division, the number of such embryos increased. Atypical cells within
the embryo underwent irregular cleavage, reminiscent of the cleavage of
parthenogenetically developing sevruga eggs. During gastrulation such
cells, when located in the region of the dorsal blastopore lip, did not
invaginate ; while at the neurula stage, when situated in the region of the
presumptive neural plate, they did not respond to the inducing action
of the underlying normal chorda-mesoderm. Later, they gradually
degenerated, while adjacent normal portions of the embryo continued
their development. In several cases achromatic mitotic figures without
chromosomes, or clearly defective aneuploid nuclei, were seen in sections
in the cells which underwent atypical cleavage. No features of acute
thermal injury of the cytoplasm, in particular vacuolization, were found
in these blastomeres. The developmental anomalies described suggest
that sturgeon fish blastomeres devoid of nuclei or those which possess
abnormal numbers of chromosomes do not participate in development.
B. Onset of Gastrulation
Apart from the formation of the normal morphogenetic properties of
embryonic cells, nuclear activity during asynchronous division is also
directly related to the onset of gastrulation. This is proved by the fact
N2
329
of adjacent normal blastomeres, while enucleated animal blastomeres
show this effect slightly, or not at all (Hörstadius et al., 1953).
A diminished competence of tissues is also found in the lethal hybrid
Rana pipiens $ χ Rana sylvatica $ (J. Moore, 1947, 1948). The ectoblast
of the early gastrula of hybrids, when transplanted to the flank of a
neurula of Rana palustris or Ambystoma
maculatum, forms nervous
tissue, sense organs, pronephros and other rudiments. Induced structures, however, appear in transplanted hybrid tissue less frequently,
they are smaller in size and less differentiated than the similar rudiments
from the ectoblast of control embryos of Rana pipiens. In a similar way,
the material of the dorsal blastopore lip of these hybrids, when transplanted to the blastocoele of the normal gastrula of Rana palustris, has a
much weaker inducing action than the dorsal blastopore lip of a normal
Rana pipiens embryo in the control. In this case secondary structures
also appear in fewer cases, they are smaller in size and more weakly
differentiated. Moore (1947) writes in this connection: 'Thus the effects
of hybridization are not confined to the factors responsible for gastrulation alone but also involve those necessary for the attainment of full
competence'.
The case of the anomalous egg development of one sevruga female
studied by Skoblina (1964) can be interpreted in the same way. Mosaics
of normal and atypical cells appeared in some eggs of this female. After a
short exposure to high temperature at the stage of the first cleavage
division, the number of such embryos increased. Atypical cells within
the embryo underwent irregular cleavage, reminiscent of the cleavage of
parthenogenetically developing sevruga eggs. During gastrulation such
cells, when located in the region of the dorsal blastopore lip, did not
invaginate ; while at the neurula stage, when situated in the region of the
presumptive neural plate, they did not respond to the inducing action
of the underlying normal chorda-mesoderm. Later, they gradually
degenerated, while adjacent normal portions of the embryo continued
their development. In several cases achromatic mitotic figures without
chromosomes, or clearly defective aneuploid nuclei, were seen in sections
in the cells which underwent atypical cleavage. No features of acute
thermal injury of the cytoplasm, in particular vacuolization, were found
in these blastomeres. The developmental anomalies described suggest
that sturgeon fish blastomeres devoid of nuclei or those which possess
abnormal numbers of chromosomes do not participate in development.
B. Onset of Gastrulation
Apart from the formation of the normal morphogenetic properties of
embryonic cells, nuclear activity during asynchronous division is also
directly related to the onset of gastrulation. This is proved by the fact
N2