108
FRED H. WILT
the entire mesoderm of the abnormal embryo could form blood island
cells.
Finnegan (1953) studied the effect of ectoderm and endoderm on
presumptive and differentiating blood island cells of some urodeles by
culturing the combined tissues in saline or coelomic fluid. The mesodermal
cells had a marked tendency to form cells of the erythrocytic series
under all conditions, but Hb was detected only when endoderm was
present in the culture. There seems little question that endoderm, while
perhaps not absolutely required, has a definite salutary effect on erythrocyte differentiation.
B. The Chick
The experimental embryology of erythrocyte formation in the chick,
while it possesses an extensive literature, is in some ways not so clear.
This stems primarily from the controversy over the mechanism of origin
of mesoderm, and the potentialities and tissue interactions present in the
unincubated blastoderm. A thorough discussion of the fate maps, and
the evidence for mechanism of mesoderm formation, has been given by
Rudnick (1955). While there is little doubt that mesoderm is the sole
source of blood island cells, recent investigations have questioned the
traditional view of origin of mesoderm from the epiblast by invagination
through the primitive streak and lateral migration of the middle germ
layer (Spratt and Haas, 1965). These authors propose that mesoderm
arises primarily from mitotic proliferation of the upper cell layer to
form a thickened middle layer, which then migrates laterally from the
streak area. The streak is analogous to a blastema in this view. This
conclusion is based on an extensive study of sectioned material, and
carbon and vital dye marking experiments. Rosenquist (1966) has recently reinvestigated the question by transplanting cell groups which
were marked prior to transplantation with thymidine-H
3
incorporated into
their DNA. His results leave little doubt that invagination through the
streak does occur and is an important (and perhaps exclusive) site of
mesoderm formation. Premesoderm cells originally located in the epiblast
pass through the streak and migrate laterally from the streak to establish the mesodermal layer. The premesoderm cells pass into the area
vasculosa. This view is in accord with the careful fate maps worked out
by Settle (1954).
A very thorough study of prospective blood island forming tissue was
made by Settle (1954) using explantation and organ culture techniques.
A summary of his results showing localization of the ability to form
erythrocytes containing Hb is shown in Fig. 4. The cells destined to form
blood islands occupy a horseshoe shaped region surrounding the posterior
and posteriolateral parts of the area pellucida; the region extends some
FRED H. WILT
the entire mesoderm of the abnormal embryo could form blood island
cells.
Finnegan (1953) studied the effect of ectoderm and endoderm on
presumptive and differentiating blood island cells of some urodeles by
culturing the combined tissues in saline or coelomic fluid. The mesodermal
cells had a marked tendency to form cells of the erythrocytic series
under all conditions, but Hb was detected only when endoderm was
present in the culture. There seems little question that endoderm, while
perhaps not absolutely required, has a definite salutary effect on erythrocyte differentiation.
B. The Chick
The experimental embryology of erythrocyte formation in the chick,
while it possesses an extensive literature, is in some ways not so clear.
This stems primarily from the controversy over the mechanism of origin
of mesoderm, and the potentialities and tissue interactions present in the
unincubated blastoderm. A thorough discussion of the fate maps, and
the evidence for mechanism of mesoderm formation, has been given by
Rudnick (1955). While there is little doubt that mesoderm is the sole
source of blood island cells, recent investigations have questioned the
traditional view of origin of mesoderm from the epiblast by invagination
through the primitive streak and lateral migration of the middle germ
layer (Spratt and Haas, 1965). These authors propose that mesoderm
arises primarily from mitotic proliferation of the upper cell layer to
form a thickened middle layer, which then migrates laterally from the
streak area. The streak is analogous to a blastema in this view. This
conclusion is based on an extensive study of sectioned material, and
carbon and vital dye marking experiments. Rosenquist (1966) has recently reinvestigated the question by transplanting cell groups which
were marked prior to transplantation with thymidine-H
3
incorporated into
their DNA. His results leave little doubt that invagination through the
streak does occur and is an important (and perhaps exclusive) site of
mesoderm formation. Premesoderm cells originally located in the epiblast
pass through the streak and migrate laterally from the streak to establish the mesodermal layer. The premesoderm cells pass into the area
vasculosa. This view is in accord with the careful fate maps worked out
by Settle (1954).
A very thorough study of prospective blood island forming tissue was
made by Settle (1954) using explantation and organ culture techniques.
A summary of his results showing localization of the ability to form
erythrocytes containing Hb is shown in Fig. 4. The cells destined to form
blood islands occupy a horseshoe shaped region surrounding the posterior
and posteriolateral parts of the area pellucida; the region extends some