A) Primitive streak
C) Holoblastic cleavage
D) Yolk plug
Correct Answer: A
Solution :In both birds and mammals during gastrula stage, certain cells of embryonic disc in its median line area, rapidly proliferate and migrate inward, forming a thick band of cells called - Primitive streak cells from this streak fan out laterally, forming a distinct cell layer between the surface layer (ectoderm) and endoderm. This layer is the mesoderm
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