THE BLOOD OF CHICK EMBRYOS
213
blood, often found in clusters of two or three cells ; later, clusters of 20 to
30 cells may occur. These clusters were considered by Fennell (1947) as
giant cells exhibiting developmental potential. Sometimes there may be
other cells, such as granulocytes or red blood cells, engulfed within the
cluster, but they can by no means have been produced by the 'giant' cell.
The embryonic thrombocytes exhibit two peculiar features. (1) They
are highly vulnerable and so they may often appear in the smear only in
the form of 'ghosts', which, however, are hardly distinguishable from
those of erythroblasts, because both forms possess a nucleus. These
ghosts are usually called 'degenerate cells' and may under some
circumstances be quite numerous in early blood smears. However, in an
excellent smear where practically no cells are damaged comparison
reveals that nearly all 'degenerate cells' must have belonged to the
thrombocyte series as revealed by the percentage of both forms. (2) The
thrombocytes during the first half of incubation possess in most cases a
round nucleus and a round cytoplasmic outline; in respect of the
nucleus, they closely resemble the definitive erythrocytes with round
nuclei (E IIj). This feature may have led Dantschakoff to call them
'Microcyten' and Fennell (1947) to call the round thrombocytes 'degenerate cells'. Their shape was explicitly stressed by Tûma (1923, 1924).
At about the 12th day 'true' oval thrombocytes appear (Fennell, 1947).
Lojda and Lemez (1955) showed by means of phase contrast microscopy and histochemical methods that from the 2nd day of incubation
thrombocytes, if present, exhibit specific morphological features which
are quite distinct from those exhibited by damaged (degenerate) red
blood cells, either primitive or definitive, so that the round thrombocytes
of the early chick embryo are already cells sui generis.
The number of thrombocytes per cu.mm during the incubation
period has not yet been estimated, because authors who estimated their
number relative to that of erythrocytes did not simultaneously determine
the erythrocyte number per cu.mm (Sugiyama, 1926; Fennell, 1947).
From the published data, and our own unpublished data, it can be
estimated that up to the 5th or 6th day of incubation, when the
primitive red blood cells dominate the blood picture, the ratio is about
3-5%, falling thereafter to a level of about 2% which is maintained
throughout the rest of the incubation period. This means that in number per cu.mm they run parallel with red blood cells, in contrast to white
blood cells. The high number of thrombocytes is the more remarkable, especially in the first half of incubation, when we realize that blood
coagulation in chick embryos does not take place before the 12th or 13th
day of incubation (Ascarelli, 1895 ; Pickering, 1925 ;Kane and Sizer, 1953).
The presence of thrombocytes was described by Delorenzi (1935) in
tissue cultures of chick embryo tissue. Three sources of origin were
213
blood, often found in clusters of two or three cells ; later, clusters of 20 to
30 cells may occur. These clusters were considered by Fennell (1947) as
giant cells exhibiting developmental potential. Sometimes there may be
other cells, such as granulocytes or red blood cells, engulfed within the
cluster, but they can by no means have been produced by the 'giant' cell.
The embryonic thrombocytes exhibit two peculiar features. (1) They
are highly vulnerable and so they may often appear in the smear only in
the form of 'ghosts', which, however, are hardly distinguishable from
those of erythroblasts, because both forms possess a nucleus. These
ghosts are usually called 'degenerate cells' and may under some
circumstances be quite numerous in early blood smears. However, in an
excellent smear where practically no cells are damaged comparison
reveals that nearly all 'degenerate cells' must have belonged to the
thrombocyte series as revealed by the percentage of both forms. (2) The
thrombocytes during the first half of incubation possess in most cases a
round nucleus and a round cytoplasmic outline; in respect of the
nucleus, they closely resemble the definitive erythrocytes with round
nuclei (E IIj). This feature may have led Dantschakoff to call them
'Microcyten' and Fennell (1947) to call the round thrombocytes 'degenerate cells'. Their shape was explicitly stressed by Tûma (1923, 1924).
At about the 12th day 'true' oval thrombocytes appear (Fennell, 1947).
Lojda and Lemez (1955) showed by means of phase contrast microscopy and histochemical methods that from the 2nd day of incubation
thrombocytes, if present, exhibit specific morphological features which
are quite distinct from those exhibited by damaged (degenerate) red
blood cells, either primitive or definitive, so that the round thrombocytes
of the early chick embryo are already cells sui generis.
The number of thrombocytes per cu.mm during the incubation
period has not yet been estimated, because authors who estimated their
number relative to that of erythrocytes did not simultaneously determine
the erythrocyte number per cu.mm (Sugiyama, 1926; Fennell, 1947).
From the published data, and our own unpublished data, it can be
estimated that up to the 5th or 6th day of incubation, when the
primitive red blood cells dominate the blood picture, the ratio is about
3-5%, falling thereafter to a level of about 2% which is maintained
throughout the rest of the incubation period. This means that in number per cu.mm they run parallel with red blood cells, in contrast to white
blood cells. The high number of thrombocytes is the more remarkable, especially in the first half of incubation, when we realize that blood
coagulation in chick embryos does not take place before the 12th or 13th
day of incubation (Ascarelli, 1895 ; Pickering, 1925 ;Kane and Sizer, 1953).
The presence of thrombocytes was described by Delorenzi (1935) in
tissue cultures of chick embryo tissue. Three sources of origin were