SERIES E, 1, 2, 3 and 4. Sections through blastoderm with a primitive
streak, towards the end of the first stage.
1. Section through the anterior part of the area pellucida. 2. Section a
little way behind 1 shewing a forward growth of mesoblast from the
primitive streak. 3. Section through primitive streak. 4. Section through
posterior part of primitive streak, shewing the great widening of primitive
streak behind.
SERIES F, 1 and 2. Sections through a blastoderm with primitive groove.
1. Section shewing a deep pit in front of primitive streak, probably an
early indication of the neurenteric canal. 2. Section immediately following
1.
FIG. G. Section through blastoderm with well-developed primitive streak,
shewing an exceptionally deep slit-like primitive groove.
SERIES H, 1 and 2. Sections through a blastoderm with a fully-developed
primitive streak.
1. Section through the anterior part of area pellucida, shewing the cubical
granular hypoblast cells in this region. 2. Section slightly behind 1,
shewing the primitive hypoblast cells differentiated into stellate cells,
which can hardly be resolved in the middle line into hypoblast and
mesoblast.