Final answer:
The endoderm of the splanchnopleure forms during gastrulation as cells from the primitive streak differentiate, migrate inwards, and displace hypoblast cells to create the layer. This endoderm then contributes to the lining of the digestive and respiratory systems, as well as other internal organs, during organogenesis.
Step-by-step explanation:
The process of gastrulation results in the development of the primitive gut and embryonic germ layers, including the endoderm, mesoderm, and ectoderm. Specifically, the endoderm of the splanchnopleure forms when cells at the primitive streak differentiate and migrate inwards. The endodermal cells displace the hypoblast and position themselves adjacent to what will become the yolk sac. During organogenesis, this layer of cells is programmed to give rise to the lining of the digestive and respiratory tracts, as well as other internal organs. In deuterostomes, the coelom also forms alongside the endoderm during enterocoely, when pouches pinched off from the endoderm expand to create the coelom.