326
T. A. DETTLAFF
even in closely related species, appears at different cleavage stages : in
Paracentrotus lividus after the 32 cell stage, in Psammechinus
miliaris
not until the 128 cell stage. The transition from synchronous to
asynchronous divisions is not related to the appearance of micromeres
since in both species they are formed at the 4th division (Agrell and
Persson, 1956). It may be suggested that the synthesis of protein precursors of the mitotic apparatus starts at the same time. According
to Mazia (1961), the supply of these protein precursors in the egg
cytoplasm allows an extremely short interphase during the period of
synchronous division ; by the end of this period the supply seems to be
exhausted.
In the interphase nuclei of the blastula, during the period of asynchronous division, nucleoli are formed; they are absent during the
period of synchrony (Richards and Porter, 1935; Richards and
Schumacher, 1935). The presence of nucleoli is a morphological sign of
synthetic processes in the interphase nuclei of the blastula (Brächet,
1944). It should be noted that Brächet (1944, 1952, 1954, 1957) has
repeatedly pointed out the significance of the blastula stage in the
metabolism of amphibian embryos (cf. also Briggs and King, 1959).
II. Interaction between the Nucleus and the Cytoplasm
during the Period of Cleavage
It was shown in Triturus palmatus (Schönmann, 1938) that a progressive decrease in the cytoplasmic volume of blastomeres during cleavage
is not at first accompanied by a corresponding decrease in nuclear volume.
It is only at the blastula stage that the normal nucleo-cytoplasmic relationship is established. From this time onwards the volumes of the
nucleus and the cytoplasm change in parallel. In Misgurnus
fossilis
(Pankova, 1963) nucleo-cytoplasmic relationships are shown to increase
sharply at the stage of the blastula and to remain constant after the
gastrula stage.
Anomalous mitoses, with non-disjunction of chromosomes and other
disturbances, appear in the diploid lethal hybrid Tr. palmatus Ç χ Salamandra maculosa S at the same stages during asynchronous divisions.
Such nuclei become pycnotic, and the cells which contain them die. The
regulation of the sizes of nuclei and their injury in lethal hybrids at the
blastula stage show that the cytoplasm at this stage affects the nuclei
more actively than at earlier stages. (There are data, however (e.g.
Baltzer, 1910) for the hybrid Strongylocentrotus
$ χ Sphaerechinus which show that instances of chromosome non-disjunction may occur
even at the very first cleavage divisions.) At the same time the appearance of a normal ratio between the sizes of the nucleus and the cyto-
T. A. DETTLAFF
even in closely related species, appears at different cleavage stages : in
Paracentrotus lividus after the 32 cell stage, in Psammechinus
miliaris
not until the 128 cell stage. The transition from synchronous to
asynchronous divisions is not related to the appearance of micromeres
since in both species they are formed at the 4th division (Agrell and
Persson, 1956). It may be suggested that the synthesis of protein precursors of the mitotic apparatus starts at the same time. According
to Mazia (1961), the supply of these protein precursors in the egg
cytoplasm allows an extremely short interphase during the period of
synchronous division ; by the end of this period the supply seems to be
exhausted.
In the interphase nuclei of the blastula, during the period of asynchronous division, nucleoli are formed; they are absent during the
period of synchrony (Richards and Porter, 1935; Richards and
Schumacher, 1935). The presence of nucleoli is a morphological sign of
synthetic processes in the interphase nuclei of the blastula (Brächet,
1944). It should be noted that Brächet (1944, 1952, 1954, 1957) has
repeatedly pointed out the significance of the blastula stage in the
metabolism of amphibian embryos (cf. also Briggs and King, 1959).
II. Interaction between the Nucleus and the Cytoplasm
during the Period of Cleavage
It was shown in Triturus palmatus (Schönmann, 1938) that a progressive decrease in the cytoplasmic volume of blastomeres during cleavage
is not at first accompanied by a corresponding decrease in nuclear volume.
It is only at the blastula stage that the normal nucleo-cytoplasmic relationship is established. From this time onwards the volumes of the
nucleus and the cytoplasm change in parallel. In Misgurnus
fossilis
(Pankova, 1963) nucleo-cytoplasmic relationships are shown to increase
sharply at the stage of the blastula and to remain constant after the
gastrula stage.
Anomalous mitoses, with non-disjunction of chromosomes and other
disturbances, appear in the diploid lethal hybrid Tr. palmatus Ç χ Salamandra maculosa S at the same stages during asynchronous divisions.
Such nuclei become pycnotic, and the cells which contain them die. The
regulation of the sizes of nuclei and their injury in lethal hybrids at the
blastula stage show that the cytoplasm at this stage affects the nuclei
more actively than at earlier stages. (There are data, however (e.g.
Baltzer, 1910) for the hybrid Strongylocentrotus
$ χ Sphaerechinus which show that instances of chromosome non-disjunction may occur
even at the very first cleavage divisions.) At the same time the appearance of a normal ratio between the sizes of the nucleus and the cyto-