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.
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.