252
RUTH BELLAIRS
According to some authors, the area vitellina consists of two zones,
the externa, whose ectoderm is often thought to be syncytial (see below),
and the interna, which is said to be derived by 'cellularization of the
syncytium'. No evidence for a syncytial region was found, however, when
the area vitellina was examined by electron microscopy (see below).
Bremer (1960) has shown that there is a correlation in early development between the extension of the mesoderm in the area opaca and the
shape, volume and density of the endoderm cells that lie beneath it. He
concluded from this that the two tissues influence each other's development. One aspect in which they appear to be independent of each other,
however, is in their mitotic rates. The mitotic rate of the endoderm is
less in the area opaca than in the area pellucida (Bremer, 1958 ; Celener,
1945). This finding is most curious, for in a rapidly expanding disc the
distal region (area opaca) would be expected to multiply at a greater
rate than the proximal one (area pellucida). Little is known, however,
about cell migrations and changes in cell shape in the area opaca.
With the appearance of the mesoderm in the area opaca the cell zone
becomes converted into the area vasculosa (Fig. 20). As the mesoderm
extends distally it invades the area vitellina interna, which itself
becomes transformed into area vasculosa. The area vasculosa thus
grows at the expense of the area vitellina.
The expansion of the blastoderm over the surface of the yolk is due
to the activity of the extreme edge itself. This has been demonstrated
by experiments in ovo (Schlesinger, 1958) and in vitro (New, 1959;
Bellairs and New, 1962).
None of these experiments, however, provides any direct evidence as
to the nature of the edge. There are two conflicting views expressed in
the literature. The first may be called the periblast theory and the
second the amoeboid movement theory, and in my opinion the latter is
correct, at least for the post-cleavage stages.
1. The Periblast Theory
The term periblast was originally used by Agassiz and Whitman ( 1884)
to describe an extra-embryonic syncytium that encircles the blastoderm
of the teleost embryo. This has been called by subsequent authors the
periblast or marginal syncytium to distinguish it from the central
syncytium, which lies beneath the fish blastoderm (see Devillers, 1961).
The periblastic syncytium can be seen by electron microscopy to be a
true syncytium in the trout egg (Bellairs, unpublished), that is, it
consists of a sheet of cytoplasm in which are embedded nuclei that are
not separated from one another by cell walls ; the cytoplasm is, however,
surrounded by a normal 'cell' membrane that separates it from the
underlying yolk (see Fig. 21 B). According to Agassiz and Whitman, the
RUTH BELLAIRS
According to some authors, the area vitellina consists of two zones,
the externa, whose ectoderm is often thought to be syncytial (see below),
and the interna, which is said to be derived by 'cellularization of the
syncytium'. No evidence for a syncytial region was found, however, when
the area vitellina was examined by electron microscopy (see below).
Bremer (1960) has shown that there is a correlation in early development between the extension of the mesoderm in the area opaca and the
shape, volume and density of the endoderm cells that lie beneath it. He
concluded from this that the two tissues influence each other's development. One aspect in which they appear to be independent of each other,
however, is in their mitotic rates. The mitotic rate of the endoderm is
less in the area opaca than in the area pellucida (Bremer, 1958 ; Celener,
1945). This finding is most curious, for in a rapidly expanding disc the
distal region (area opaca) would be expected to multiply at a greater
rate than the proximal one (area pellucida). Little is known, however,
about cell migrations and changes in cell shape in the area opaca.
With the appearance of the mesoderm in the area opaca the cell zone
becomes converted into the area vasculosa (Fig. 20). As the mesoderm
extends distally it invades the area vitellina interna, which itself
becomes transformed into area vasculosa. The area vasculosa thus
grows at the expense of the area vitellina.
The expansion of the blastoderm over the surface of the yolk is due
to the activity of the extreme edge itself. This has been demonstrated
by experiments in ovo (Schlesinger, 1958) and in vitro (New, 1959;
Bellairs and New, 1962).
None of these experiments, however, provides any direct evidence as
to the nature of the edge. There are two conflicting views expressed in
the literature. The first may be called the periblast theory and the
second the amoeboid movement theory, and in my opinion the latter is
correct, at least for the post-cleavage stages.
1. The Periblast Theory
The term periblast was originally used by Agassiz and Whitman ( 1884)
to describe an extra-embryonic syncytium that encircles the blastoderm
of the teleost embryo. This has been called by subsequent authors the
periblast or marginal syncytium to distinguish it from the central
syncytium, which lies beneath the fish blastoderm (see Devillers, 1961).
The periblastic syncytium can be seen by electron microscopy to be a
true syncytium in the trout egg (Bellairs, unpublished), that is, it
consists of a sheet of cytoplasm in which are embedded nuclei that are
not separated from one another by cell walls ; the cytoplasm is, however,
surrounded by a normal 'cell' membrane that separates it from the
underlying yolk (see Fig. 21 B). According to Agassiz and Whitman, the