At the beginning of the fetal period, the pons starts to bulge ventrally to form the pontine flexure. By S12, the vesicle of the inner ear (vesicula otica) detaches from the ectoderm; shortly afterwards, it starts to differentiate.
The trigeminal nerve and the facial nerve are the first cranial nerves of the rhombencephalon to form, at S12. By S13, the immense trigeminal ganglion and the three branches of the trigeminal nerve can be distinguished. The trigeminal nerve marks the pontine flexure. The next cranial nerves to appear are the glossopharyngeal nerve, the vagus nerve and the accessory nerve. At S14, the hypoglossal nerve appears. The abducens nerve is the last to appear; this occurs at S15/16.