CELL DIVISIONS
331
corresponding to a more clearly expressed differentiation of the cytoplasm, morphogenetic function of the nuclei starts much earlier than in
'regulation' eggs (Neyfakh, 1961).
Thus, for example, the onset of nuclear function in Limnaea stagnalis
can be observed from the 12-16 cell stage (Neyfakh, 1961). From the
viewpoint of the problem under discussion, it is of importance that the
transition to asynchronous division in Limnaea (Wierzejski, 1905) and
the appearance of typical interphase nuclei with nucleoli (Raven, 1948)
both occur at the 16 cell stage.
On the basis of a comparison of his own data on the time of onset of
nuclear function and the time of manifestation of nuclear activity with
data on the distribution of cytoplasmic components during cleavage in
molluscs and nematodes, Neyfakh (1961) showed that the onset of
nuclear function in mosaic eggs takes place directly after the cytoplasmic egg zones, which give rise to the main rudiments of the embryo,
become localized in different blastomeres (mesoblasts, ectoblasts and
entoblasts). Thus in the case of the 'mosaic' developmental type there
exists a clearly expressed correlation between the number of cell
divisions providing the level of cytoplasmic segregation upon which the
functioning of the nuclei begins, and the processes of differentiation.
It can be thought that the onset of nuclear function in 'regulation'
eggs also coincides with a certain level of cytoplasmic segregation ; in
particular, probably, with the distribution of different portions of the
region of the grey crescent into individual blastomeres. Corresponding
to the weaker expression of the differentiation of cytoplasmic zones in
'regulation' eggs than in 'mosaic' ones, the period from the onset of
nuclear functioning in the former is also more prolonged.
At any rate, until data are available which provide evidence of major
differences between nuclear stimulation in 'regulation' eggs and in
'mosaic' ones, it is natural to suggest that the onset of nuclear function
in each type corresponds in time to certain cleavage stages.
In this connection the problem of the consistency of the number of
cleavage divisions up to the onset of gastrulation, and of the consistency
of the relative duration of the cleavage period (up to the onset of
gastrulation) and of subsequent developmental periods in different
individuals of the same animal species under the same and different
temperatures (within the optimal range), as well as their relationships in
different animal species, is of great interest.
IV. The Ratio of the Duration of Different Developmental Periods
A. Duration of Cleavage
The simple counting of the cells in an embryo at the onset of gastrulation would give a direct answer to the question of their number.
331
corresponding to a more clearly expressed differentiation of the cytoplasm, morphogenetic function of the nuclei starts much earlier than in
'regulation' eggs (Neyfakh, 1961).
Thus, for example, the onset of nuclear function in Limnaea stagnalis
can be observed from the 12-16 cell stage (Neyfakh, 1961). From the
viewpoint of the problem under discussion, it is of importance that the
transition to asynchronous division in Limnaea (Wierzejski, 1905) and
the appearance of typical interphase nuclei with nucleoli (Raven, 1948)
both occur at the 16 cell stage.
On the basis of a comparison of his own data on the time of onset of
nuclear function and the time of manifestation of nuclear activity with
data on the distribution of cytoplasmic components during cleavage in
molluscs and nematodes, Neyfakh (1961) showed that the onset of
nuclear function in mosaic eggs takes place directly after the cytoplasmic egg zones, which give rise to the main rudiments of the embryo,
become localized in different blastomeres (mesoblasts, ectoblasts and
entoblasts). Thus in the case of the 'mosaic' developmental type there
exists a clearly expressed correlation between the number of cell
divisions providing the level of cytoplasmic segregation upon which the
functioning of the nuclei begins, and the processes of differentiation.
It can be thought that the onset of nuclear function in 'regulation'
eggs also coincides with a certain level of cytoplasmic segregation ; in
particular, probably, with the distribution of different portions of the
region of the grey crescent into individual blastomeres. Corresponding
to the weaker expression of the differentiation of cytoplasmic zones in
'regulation' eggs than in 'mosaic' ones, the period from the onset of
nuclear functioning in the former is also more prolonged.
At any rate, until data are available which provide evidence of major
differences between nuclear stimulation in 'regulation' eggs and in
'mosaic' ones, it is natural to suggest that the onset of nuclear function
in each type corresponds in time to certain cleavage stages.
In this connection the problem of the consistency of the number of
cleavage divisions up to the onset of gastrulation, and of the consistency
of the relative duration of the cleavage period (up to the onset of
gastrulation) and of subsequent developmental periods in different
individuals of the same animal species under the same and different
temperatures (within the optimal range), as well as their relationships in
different animal species, is of great interest.
IV. The Ratio of the Duration of Different Developmental Periods
A. Duration of Cleavage
The simple counting of the cells in an embryo at the onset of gastrulation would give a direct answer to the question of their number.