Final answer:
Sea urchin primary mesenchyme cells originate from the mesoderm and move inward, crawling along collagen fibers which are a component of the extracellular matrix.
Step-by-step explanation:
The question focuses on the development of sea urchin primary mesenchyme cells and where they move during embryonic development. In the development of a sea urchin, after several rounds of cell division post-fertilization, the embryo forms a structure known as the blastula. During gastrulation, certain cells in the blastula rearrange themselves to form three primary layers of cells: the ectoderm, mesoderm, and endoderm. Among these, the primary mesenchyme cells originate from the mesoderm. These cells become motile and leave the epithelium, moving inward. The primary mesenchyme cells crawl along extracellular matrix components, such as collagen fibers (which is the correct answer from the options provided), to reach their destinations within the embryo where they will contribute to forming various structures, including the sea urchin's spicules.