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FIG. 3. (a) Transverse section through the postaxial part of the fore-limb bud of a 10-day
mouse embryo. Alkaline phosphatase (Gomori's technique). Positive reaction in the whole
thickened ventral ectoderm and in the differentiating a.e.r. (b) Transverse section through
the axial part of the fore-limb bud of a mole embryo (about 8 mm.). Alkaline phosphatase (v. Kossa-Barger's technique). Enzyme present in the axillary zone of the
ectoderm, in the differentiating a.e.r. and in the proximal mesoderm. (c) Transverse
section through the postaxial part of the fore-limb bud of a 10J-day mouse embryo.
Acid phosphatase (Gomori's technique). Acid phosphatase present in the axillary ectoderm, in the a.e.r. and in the mesoderm (except in its outer zone), (d) Transverse section
through the postaxial part of the fore-limb bud of a ΙΗ-day rat embryo. Acid phosphatase (Gomori's technique). Strong reaction in the basal layers of the a.e.r. a.e.—
axillary ectoderm; a.e.r.—apical ectodermal ridge; m.v.s.—marginal
venous sinus;
t.v.e.—thickened ventral ectoderm.