The path of migration of primordial germ cells (PGCs) is an example of chemotaxis. In zebrafish embryos, the PGCs follow the gradient of SDF1 by expressing the CXCR4 receptor. Fill in the blanks with the best answers.
a. differentiation, CXCR7, Frizzled
b. determination, SDF1, Notch
c. chemotaxis, bindin, CXCR7
d. chemotaxis, SDF1, CXCR4