Early on the seventh day the embryonic area becomes pyriform, and at its posterior and narrower end a primitive streak makes its appearance, which is due to a proliferation of rounded cells from the epiblast. At the time when this proliferation commences the layer of hypoblastic mesoblast is present, especially just in front of, and at the sides of, the anterior part of the streak; but no mesoblast is found in the anterior part of the embryonic area. These features are shewn in [fig. 138] A and B. The mesoblast derived from the proliferation of the epiblast soon joins the mesoblast already present; though in many sections it seems possible to trace a separation between the two parts ([fig. 139] B) of the mesoblast.
Fig. 138. Two sections through oval blastoderms of a Rabbit on the seventh day. The length of the area was about 1.2 mm. and its breadth about .86 mm.
A. Through the region of the blastoderm in front of the primitive streak; B. through the front part of the primitive streak; ep. epiblast; m. mesoblast; hy. hypoblast; pr. primitive streak.
Fig. 139. Two transverse sections through the embryonic area of an embryo Rabbit of seven days.
The embryo has nearly the structure represented in fig. 140.
A. is taken through the anterior part of the embryonic area. It represents about half the breadth of the area, and there is no trace of a medullary groove or of the mesoblast.
B. Is taken through the posterior part of the primitive streak.
ep. epiblast; hy. hypoblast.
During the seventh day the primitive streak becomes a more pronounced structure, the mesoblast in its neighbourhood increases in quantity, while an axial groove—the primitive groove—is formed on its upper surface. The mesoblastic layer in front of the primitive streak becomes thicker, and, in the two-layered region in front, the epiblast becomes several rows deep ([fig. 139] A).
