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FRED H. WILT
fairly definite region, one which will give rise to the area opaca vasculosa,
is more or less firmly committed to red blood cell differentiation; i.e.,
blood cell forming potency is "determined." Whether this determined
state can be shifted, like "dorsalization" in amphibia, is not known; but
obviously some profound, if not irreversible, choices have been made by
blastoderm cells.
The exact position of prospective blood island cells and their developmental plasticity in the unincubated blastoderm is not completely clear,
but the different regions of the blastoderm are certainly not completely
equivalent (Settle, 1954). The developmental age of unincubated blastoderms is difficult to assess and may vary widely. Furthermore, all marking experiments on these youngest stages of development have only
marked surface cell movements.
The state of determination and fate map of unincubated blastoderm
with respect to Hb formation can be evaluated from scattered observations reported by Spratt and Haas (1960a-c), and some unpublished
experiments on this matter have been carried out in my laboratory. In
unincubated blastoderms, the junction between the "yolky" cells of the
area opaca and the central clear cells overlying the subgerminal cavity
is rather conspicuous in freshly dissected blastoderms, and Spratt terms
this boundary the junction zone. Just medial to it is the marginal zone.
The "normal" fate of the junction zone is formation of the area vasculosa
(Spratt and Haas, 1960a). This conclusion is based on carbon marking
of the lower layer, junction zone cells always forming area vasculosa.
Experiments involving isolation of blastoderm parts (Spratt and Haas,
1960c) also localized potentiality for erythrocyte formation in the junction zone and marginal zone. There is some possibility that the early
area pellucida may be transformed into area vasculosa. The boundaries
of the marginal zone may be diffuse, and its determination not fixed.
Based on these findings one may say that the first erythrocytes are
apparently derived from cells in a circular or semicircular area (the
capacity to form blood cells being much weaker anteriorly) of the junction zone-marginal zone level. Small pieces of unincubated blastoderms
(one-sixteenth of a blastoderm) have been isolated and cultured, in our
laboratory, and they show a distribution of the Hb forming potentiality
similar to this horseshoe shape.
The necessary role of coordinated cell movements in the blastoderm
for the formation of area vasculosa has been stressed by Spratt and
Haas (1960b). If lower cell movements are inhibited by explantation of
lower surface against agar or by placing large amounts of carmine
powder on the lower surface, an area vasculosa does not form in many
instances, but whether some Hb eventually formed was not noted by
Spratt. In many cases, a reduced area vasculosa does form, which they
FRED H. WILT
fairly definite region, one which will give rise to the area opaca vasculosa,
is more or less firmly committed to red blood cell differentiation; i.e.,
blood cell forming potency is "determined." Whether this determined
state can be shifted, like "dorsalization" in amphibia, is not known; but
obviously some profound, if not irreversible, choices have been made by
blastoderm cells.
The exact position of prospective blood island cells and their developmental plasticity in the unincubated blastoderm is not completely clear,
but the different regions of the blastoderm are certainly not completely
equivalent (Settle, 1954). The developmental age of unincubated blastoderms is difficult to assess and may vary widely. Furthermore, all marking experiments on these youngest stages of development have only
marked surface cell movements.
The state of determination and fate map of unincubated blastoderm
with respect to Hb formation can be evaluated from scattered observations reported by Spratt and Haas (1960a-c), and some unpublished
experiments on this matter have been carried out in my laboratory. In
unincubated blastoderms, the junction between the "yolky" cells of the
area opaca and the central clear cells overlying the subgerminal cavity
is rather conspicuous in freshly dissected blastoderms, and Spratt terms
this boundary the junction zone. Just medial to it is the marginal zone.
The "normal" fate of the junction zone is formation of the area vasculosa
(Spratt and Haas, 1960a). This conclusion is based on carbon marking
of the lower layer, junction zone cells always forming area vasculosa.
Experiments involving isolation of blastoderm parts (Spratt and Haas,
1960c) also localized potentiality for erythrocyte formation in the junction zone and marginal zone. There is some possibility that the early
area pellucida may be transformed into area vasculosa. The boundaries
of the marginal zone may be diffuse, and its determination not fixed.
Based on these findings one may say that the first erythrocytes are
apparently derived from cells in a circular or semicircular area (the
capacity to form blood cells being much weaker anteriorly) of the junction zone-marginal zone level. Small pieces of unincubated blastoderms
(one-sixteenth of a blastoderm) have been isolated and cultured, in our
laboratory, and they show a distribution of the Hb forming potentiality
similar to this horseshoe shape.
The necessary role of coordinated cell movements in the blastoderm
for the formation of area vasculosa has been stressed by Spratt and
Haas (1960b). If lower cell movements are inhibited by explantation of
lower surface against agar or by placing large amounts of carmine
powder on the lower surface, an area vasculosa does not form in many
instances, but whether some Hb eventually formed was not noted by
Spratt. In many cases, a reduced area vasculosa does form, which they