CELL DIVISIONS
335
degrees of ploidy, the duration of the period up to the onset of gastrulation will be the same.
It has been shown that haploid embryos of Rana pipiens, obtained
without injury to the cytoplasm by the removal of the female pronuclei
after insemination (Porter, 1939), undergo cleavage with the same
rhythm as diploids (the cleavage rhythm, as shown in the paper of
A. R. Moore, 1933, is determined by the cytoplasm and not by the
nucleus). Gastrulation in haploid embryos, however, begins regularly
one hour later than in diploid embryos at the same temperature. This
delay in the haploide is maintained throughout development.
R. pipiens triploids obtained by temperature shock (Briggs, 1947),
like the haploide undergo cleavage with the same rhythm as diploids.
Their mitotic index is also the same at all the developmental stages
studied (from the neurula). At the gastrula stage the triploids are
distinguished by a relatively greater size of cell than the diploids.
The regulation of the sizes and the number of cells takes place,
according to Briggs, during the first growth interphases; in triploids
the growth of the cells proceeds for longer and they reach larger sizes.
Briggs writes : Ά suitable prolongation of the first "growth interphase"
would regulate cell size and cell number in one step'. However, the
regulation of cell number in this way would take place only if gastrulation begins somewhat earlier.
As to the suggestion of Briggs that the regulation of size is realized
through an increase in growth of the cytoplasm, it seems to be insufficiently based, since the cytoplasm does not grow at these stages. It is
more likely that, as in the development of diploid embryos, regulation
is realized by means of a change in the volume of the nucleus as well as
by means of a shift of the onset of gastrulation to earlier cleavage
stages.
A similar hypothesis was put forth by Ruhland (1958), though this
author advanced another interpretation of the causes bringing about
the shift of the onset of gastrulation. The problem requires further
cytological investigation. Since the cells undergo asynchronous division
at the blastula stage, when a shift of the onset of gastrulation occurs the
number of cells can change not only in a ratio divisible by two, as in
haploids and tetraploids, but in other ratios as well, such as by one and
a half times, as in triploids. The main argument put forward by
Fankhauser (1952) against the suggestion that cell size is regulated
through a change in the number of cleavage divisions before the onset
of gastrulation, was based on the supposition that in this way regulation
is possible only in a ratio divisible by two ; this argument does not hold
for the regulation of cell number.
It can therefore be suggested that the mechanism by which changes
335
degrees of ploidy, the duration of the period up to the onset of gastrulation will be the same.
It has been shown that haploid embryos of Rana pipiens, obtained
without injury to the cytoplasm by the removal of the female pronuclei
after insemination (Porter, 1939), undergo cleavage with the same
rhythm as diploids (the cleavage rhythm, as shown in the paper of
A. R. Moore, 1933, is determined by the cytoplasm and not by the
nucleus). Gastrulation in haploid embryos, however, begins regularly
one hour later than in diploid embryos at the same temperature. This
delay in the haploide is maintained throughout development.
R. pipiens triploids obtained by temperature shock (Briggs, 1947),
like the haploide undergo cleavage with the same rhythm as diploids.
Their mitotic index is also the same at all the developmental stages
studied (from the neurula). At the gastrula stage the triploids are
distinguished by a relatively greater size of cell than the diploids.
The regulation of the sizes and the number of cells takes place,
according to Briggs, during the first growth interphases; in triploids
the growth of the cells proceeds for longer and they reach larger sizes.
Briggs writes : Ά suitable prolongation of the first "growth interphase"
would regulate cell size and cell number in one step'. However, the
regulation of cell number in this way would take place only if gastrulation begins somewhat earlier.
As to the suggestion of Briggs that the regulation of size is realized
through an increase in growth of the cytoplasm, it seems to be insufficiently based, since the cytoplasm does not grow at these stages. It is
more likely that, as in the development of diploid embryos, regulation
is realized by means of a change in the volume of the nucleus as well as
by means of a shift of the onset of gastrulation to earlier cleavage
stages.
A similar hypothesis was put forth by Ruhland (1958), though this
author advanced another interpretation of the causes bringing about
the shift of the onset of gastrulation. The problem requires further
cytological investigation. Since the cells undergo asynchronous division
at the blastula stage, when a shift of the onset of gastrulation occurs the
number of cells can change not only in a ratio divisible by two, as in
haploids and tetraploids, but in other ratios as well, such as by one and
a half times, as in triploids. The main argument put forward by
Fankhauser (1952) against the suggestion that cell size is regulated
through a change in the number of cleavage divisions before the onset
of gastrulation, was based on the supposition that in this way regulation
is possible only in a ratio divisible by two ; this argument does not hold
for the regulation of cell number.
It can therefore be suggested that the mechanism by which changes