Final answer:
The cecum develops from the archenteron during the embryonic developmental stage known as gastrulation, and it is part of the embryonic gut derived from the endoderm.
Step-by-step explanation:
The originating developmental primitive structure of the cecum is the archenteron, which is formed during the early embryonic phase known as gastrulation. The cecum arises from the embryonic gut, specifically from the endoderm layer. During gastrulation, a third layer, the mesoderm, also develops, contributing to various body structures. The gut itself forms from the primitive gut (archenteron), with the blastopore giving rise to the anus or mouth, depending on whether the organism is a deuterostome or a protostome. In the case of humans, who are deuterostomes, the mouth forms opposite from the blastopore, and the cecum, as the first part of the large intestine, develops as the digestive tract elaborates post-gastrulation.