254
R U T H B E L L A I R S
The supporting evidence in favour of it was derived from a microscopical
examination of the edge (Bellairs, 1963). The margin of overgrowth was
found to consist of a single layer of cells. This is usually regarded as
ectodermal since it is continuous proximally with the ectoderm of the
area vitellina. These cells are, however, completely different from the
cells seen in the rest of the blastoderm in that they possess a cell body
that is usually extended on its distal side into a long process or processes
(Fig. 22). Individual processes may be as much as 500 μ long, though as
FIG. 22. Diagram of a section through the edge of a chick blastoderm based on electron micrographs. No periblast is present, and the peripheral cells are extended into long
amoeboid-like processes. (From Bellairs, 1963.)
little as 0·25 μ deep, and together they form a fringe-like border around
the whole blastoderm. New (1959) showed experimentally that the edge
of the blastoderm had a special affinity for the inner side of the vitelline
membrane. The strength of this attachment is shown by the fact that it
persists even during the procedure of preparing material for electron
microscopy. I t seems likely that these long processes attach themselves
to the vitelline membrane and then contract, thus dragging the blastoderm with them.
Although no free (periblastic) nuclei were seen at the extreme edge of
the blastoderm, the endoderm of the region immediately proximal to it,
the so-called syncytial zone of the area vitellina, was examined to see if a
periblastic syncytium existed there. I t was, however, found by electron
microscopy that the endoderm consisted of well-defined, discrete cells,
each having a single nucleus and being surrounded by a normal cell
R U T H B E L L A I R S
The supporting evidence in favour of it was derived from a microscopical
examination of the edge (Bellairs, 1963). The margin of overgrowth was
found to consist of a single layer of cells. This is usually regarded as
ectodermal since it is continuous proximally with the ectoderm of the
area vitellina. These cells are, however, completely different from the
cells seen in the rest of the blastoderm in that they possess a cell body
that is usually extended on its distal side into a long process or processes
(Fig. 22). Individual processes may be as much as 500 μ long, though as
FIG. 22. Diagram of a section through the edge of a chick blastoderm based on electron micrographs. No periblast is present, and the peripheral cells are extended into long
amoeboid-like processes. (From Bellairs, 1963.)
little as 0·25 μ deep, and together they form a fringe-like border around
the whole blastoderm. New (1959) showed experimentally that the edge
of the blastoderm had a special affinity for the inner side of the vitelline
membrane. The strength of this attachment is shown by the fact that it
persists even during the procedure of preparing material for electron
microscopy. I t seems likely that these long processes attach themselves
to the vitelline membrane and then contract, thus dragging the blastoderm with them.
Although no free (periblastic) nuclei were seen at the extreme edge of
the blastoderm, the endoderm of the region immediately proximal to it,
the so-called syncytial zone of the area vitellina, was examined to see if a
periblastic syncytium existed there. I t was, however, found by electron
microscopy that the endoderm consisted of well-defined, discrete cells,
each having a single nucleus and being surrounded by a normal cell