SERIES I, 1, 2, 3, 4 and 5. Sections through blastoderm somewhat older than
Series H.
1. Section through area pellucida well in front of primitive streak. 2.
Section through area pellucida just in front of primitive streak. 3.
Section through the front end of primitive streak. 4. Section slightly
behind 3. 5. Section slightly behind 4.
SERIES K, 1, 2, 3, 4 and 5. Sections through a blastoderm in which the
first traces of notochord and medullary groove have made their appearance.
Rather more than half the section is represented in each figure, but the
right half is represented in 1 and 3, and the left in 2 and 4.
1. Section through notochord immediately behind the head fold. 2. Section
shewing medullary groove a little behind 1. 3. Section just in front of the
primitive streak. 4 and 5. Sections through the front end of the primitive
streak.
FIG. L. Surface view of blastoderm with a very young primitive streak.