SERIES A, 1 and 2. Sections through the blastoderm before the appearance of
primitive streak.
1. Section through anterior part of area pellucida in front of embryonic
shield. The hypoblast here forms an imperfect layer. The figure represents
about half the section. 2. Section through same blastoderm, in the region
of the embryonic shield. Between the epiblast and hypoblast are a number of
undifferentiated cells. The figure represents considerably more than half
the section.
SERIES B, 1, 2 and 3. Sections through a blastoderm with a very young
primitive streak.
1. Section through the anterior part of the area pellucida in front of the
primitive streak. 2. Section through about the middle of the primitive
streak. 3. Section through the posterior part of the primitive streak.
SERIES C, 1 and 2. Sections through a blastoderm with a young primitive
streak.
1. Section through the front end of the primitive streak. 2. Section
through the primitive streak, somewhat behind 1. Both figures shew very
clearly the difference in character between the cells of the epiblastic
mesoblast of the primitive streak, and the more granular cells of the
mesoblast derived from the hypoblast.
FIG. D. Longitudinal section through the axial line of the primitive
streak, and the part of the blastoderm in front of it, of an embryo duck
with a well-developed primitive streak.