Which of the following is a development of the mesoderm during the Embryonic Period?

The development of the blood and vascular system is indeed a significant product of the mesoderm during the Embryonic Period. The mesoderm is one of the three primary germ layers formed during embryogenesis, the others being the ectoderm and endoderm.

Blood cell formation, or hematopoiesis, begins in the mesodermal layer, specifically in areas such as the yolk sac, and later continues in the liver and bone marrow as development progresses. Additionally, the vascular system, which includes the formation of blood vessels, is derived from mesodermal structures known as angioblasts.

In contrast, the epidermis is formed from the ectoderm, which primarily gives rise to structures associated with the skin and nervous system. The endometrium, which is the lining of the uterus, is derived from the endoderm, specifically from the uterine mesoderm. The salivary glands also originate from the ectoderm. This distinction underscores the importance of the mesoderm in developing a range of vital structures, particularly the circulatory system during the critical stages of embryonic development.