Final answer:
The anterior lymph sacs and posterior lymph sacs are not among the primary lymph sacs that form during the sixth week of embryonic development.
Step-by-step explanation:
The primary lymph sacs that form during the sixth week of development include the jugular lymph sacs, the retroperitoneal lymph sac, and the cisterna chyli. The anterior lymph sacs and posterior lymph sacs are not among the primary lymph sacs that develop at this stage. Specifically, the cisterna chyli is a sac-like chamber that receives lymph from the lower abdomen, pelvis, and lower limbs and is part of the lymphatic system. Primary lymphoid tissue meanwhile includes bone marrow and the thymus, and secondary lymphoid tissues include the spleen and lymph nodes, which play crucial roles in filtering lymph and housing lymphocytes for the immune response.