256
RUTH BELLAIRS
membrane. These cells are, however, separated from one another by a
meshwork of fine membranes in which small patches of yolk are trapped
(Figs. 9 and 23). This meshwork extends beyond the boundary of the
true blastoderm edge and thus corresponds in its location with the
region described in the literature as periblast.
The presence of the membranes between the cells makes it difficult to
decide from light microscopy exactly where the boundary of each cell
lies, so that the general impression is gained that no proper cell membranes exist. The nuclei, therefore, appear by light microscopy to be
separated by a vacuolated cytoplasm in which yolk is trapped, i.e. a
periblastic syncytium.
There are several reasons for believing that the meshwork of membranes is formed by hydration of yolk. Firstly, there is evidence that
during early development fluid (mainly water) passes from the albumen
through the vitelline membrane and the blastoderm into the yolk (New,
1956; Needham, 1950; Mclndoe, 1960); centrally, beneath the blastoderm, this water forms the sub-blastodermic fluid, but peripherally it
probably extends for a few microns into the yolk. Secondly, myelin
figures can be produced from yolk (see Section I I , C) and there is no
reason to suppose that hydration of this type cannot occur in ovo.
Thirdly, the fine structure of the yolk membranes (Fig. 9) closely resembles that of myelin figures as described by Stoeckenius (1959) and
Revel eiaZ. (1958).
Thus it appears that the endoderm of the area vitellina consists of
clearly defined cells separated from one another by patches of yolk
which have been modified to form myelin figures. I t does not consist of a
periblastic syncytium, at least in the post-cleavage stages. Hence the
classical view of the mode of proliferation of the area opaca now seems
to be incorrect.
B. Theories on the Origin of Cells or Nuclei from Yolk
Before the term 'periblast' was applied to the area vitellina it was
known as the parablast by His (1875) who considered it to be a region
where nuclei arose de novo from yolk. Agassiz and Whitman (1884)
disagreed with this concept, at least in fishes, and introduced the term
periblast in an attempt to clarify the issue.
I have been unable to find any convincing evidence in the literature
that either nuclei or cells may arise directly from yolk. A number of
investigators have, however, suggested the possibility, mainly as a result
of histological studies (e.g. Lavodsky, 1901; Golgi, 1923; Pierrantoni,
1951 ; Lepeschinskaya, 1954; Jordanov and Georgiev, 1957). According
to Roskin (1955) these misconceptions may have arisen in some cases
because of the technical difficulties involved in staining the material,
RUTH BELLAIRS
membrane. These cells are, however, separated from one another by a
meshwork of fine membranes in which small patches of yolk are trapped
(Figs. 9 and 23). This meshwork extends beyond the boundary of the
true blastoderm edge and thus corresponds in its location with the
region described in the literature as periblast.
The presence of the membranes between the cells makes it difficult to
decide from light microscopy exactly where the boundary of each cell
lies, so that the general impression is gained that no proper cell membranes exist. The nuclei, therefore, appear by light microscopy to be
separated by a vacuolated cytoplasm in which yolk is trapped, i.e. a
periblastic syncytium.
There are several reasons for believing that the meshwork of membranes is formed by hydration of yolk. Firstly, there is evidence that
during early development fluid (mainly water) passes from the albumen
through the vitelline membrane and the blastoderm into the yolk (New,
1956; Needham, 1950; Mclndoe, 1960); centrally, beneath the blastoderm, this water forms the sub-blastodermic fluid, but peripherally it
probably extends for a few microns into the yolk. Secondly, myelin
figures can be produced from yolk (see Section I I , C) and there is no
reason to suppose that hydration of this type cannot occur in ovo.
Thirdly, the fine structure of the yolk membranes (Fig. 9) closely resembles that of myelin figures as described by Stoeckenius (1959) and
Revel eiaZ. (1958).
Thus it appears that the endoderm of the area vitellina consists of
clearly defined cells separated from one another by patches of yolk
which have been modified to form myelin figures. I t does not consist of a
periblastic syncytium, at least in the post-cleavage stages. Hence the
classical view of the mode of proliferation of the area opaca now seems
to be incorrect.
B. Theories on the Origin of Cells or Nuclei from Yolk
Before the term 'periblast' was applied to the area vitellina it was
known as the parablast by His (1875) who considered it to be a region
where nuclei arose de novo from yolk. Agassiz and Whitman (1884)
disagreed with this concept, at least in fishes, and introduced the term
periblast in an attempt to clarify the issue.
I have been unable to find any convincing evidence in the literature
that either nuclei or cells may arise directly from yolk. A number of
investigators have, however, suggested the possibility, mainly as a result
of histological studies (e.g. Lavodsky, 1901; Golgi, 1923; Pierrantoni,
1951 ; Lepeschinskaya, 1954; Jordanov and Georgiev, 1957). According
to Roskin (1955) these misconceptions may have arisen in some cases
because of the technical difficulties involved in staining the material,