EMBRYONIC HEMOGLOBIN SYNTHESIS
111
ascribe to some residual coordinated cell movements in the lower layer.
In over 100 unincubated blastoderms explanted with the lower layer
against the agar, red blood cells always formed and were found in a
horseshoe-shaped ring in the area opaca (Wilt, unpublished results) ; no
axis formation nor apparent morphogenesis was present. Since even very
small isolates can form Hb, it is doubtful if extensive and coordinated
blastoderm cell movements are required for Hb formation, although the
movements may be necessary for normal tissue architecture to form in
the area vasculosa.
A fact which makes interpretation difficult is that the marginal zone
apparently possesses diverse differentiative potentialities. Spratt and
Haas (1960c) have discussed experiments showing a great regulative
potentiality of any part of the unincubated blastoderm containing some
marginal zone cells. If one ascribes a regulative, organizing, integrative
capacity to the marginal zone, it is possible that particular cell groups
of the unincubated blastoderm are not "determined" to form erythrocytes, but that the organizing capacities of the area are necessary to
induce the first early commitments of prospective erythroblasts along
this course. Perhaps it is best to conclude simply that the regions of
the marginal zone eventually give rise to erythrocytes. Whether interactions between cells of different tissues (heterotypic) or interactions
between similar cells (homotypic) are occurring at these stages is an
intriguing and difficult question which deserves study (cf. Grobstein,
1962).
There are some suggestions that specific tissue associations do play a
role in the subsequent formation of the first red blood cells. Sabin (1920)
and Romanoff (1960) have adequately reviewed the cytology and histology of the intermediate steps in morphogenesis of the primitive
vascular system. In the mesoderm of the area opaca vasculosa, obvious
condensations of cells can be seen in the splanchopleuric mesoderm
layer in close association with the underlying yolk-filled endoderm. The
condensations, called blood islands by some (although Sabin prefers the
term angioblasts and reserves "blood islands" for groups of erythroblasts actually containing Hb) may be seen by the head-process stage.
Mato et al. (1964) have studied the close association of blood islands
with the endoderm using the electron microscope. In 3-somite blastoderms, the blood island and endoderm were quite close, but no processes
extended between them; by the 4-somite stage, projections from endoderm cells touched the mesoderm. Electron dense material was seen to
accumulate in the interspace between the two tissue layers.
Wilt (1965b) studied the effect of separation of endoderm from the
mesodermal cells (which usually remained attached to ectoderm). If any
mesoderm cells remained attached to endoderm, normal blood island
111
ascribe to some residual coordinated cell movements in the lower layer.
In over 100 unincubated blastoderms explanted with the lower layer
against the agar, red blood cells always formed and were found in a
horseshoe-shaped ring in the area opaca (Wilt, unpublished results) ; no
axis formation nor apparent morphogenesis was present. Since even very
small isolates can form Hb, it is doubtful if extensive and coordinated
blastoderm cell movements are required for Hb formation, although the
movements may be necessary for normal tissue architecture to form in
the area vasculosa.
A fact which makes interpretation difficult is that the marginal zone
apparently possesses diverse differentiative potentialities. Spratt and
Haas (1960c) have discussed experiments showing a great regulative
potentiality of any part of the unincubated blastoderm containing some
marginal zone cells. If one ascribes a regulative, organizing, integrative
capacity to the marginal zone, it is possible that particular cell groups
of the unincubated blastoderm are not "determined" to form erythrocytes, but that the organizing capacities of the area are necessary to
induce the first early commitments of prospective erythroblasts along
this course. Perhaps it is best to conclude simply that the regions of
the marginal zone eventually give rise to erythrocytes. Whether interactions between cells of different tissues (heterotypic) or interactions
between similar cells (homotypic) are occurring at these stages is an
intriguing and difficult question which deserves study (cf. Grobstein,
1962).
There are some suggestions that specific tissue associations do play a
role in the subsequent formation of the first red blood cells. Sabin (1920)
and Romanoff (1960) have adequately reviewed the cytology and histology of the intermediate steps in morphogenesis of the primitive
vascular system. In the mesoderm of the area opaca vasculosa, obvious
condensations of cells can be seen in the splanchopleuric mesoderm
layer in close association with the underlying yolk-filled endoderm. The
condensations, called blood islands by some (although Sabin prefers the
term angioblasts and reserves "blood islands" for groups of erythroblasts actually containing Hb) may be seen by the head-process stage.
Mato et al. (1964) have studied the close association of blood islands
with the endoderm using the electron microscope. In 3-somite blastoderms, the blood island and endoderm were quite close, but no processes
extended between them; by the 4-somite stage, projections from endoderm cells touched the mesoderm. Electron dense material was seen to
accumulate in the interspace between the two tissue layers.
Wilt (1965b) studied the effect of separation of endoderm from the
mesodermal cells (which usually remained attached to ectoderm). If any
mesoderm cells remained attached to endoderm, normal blood island