rhombencephalon into its final parts. The final parts are defined as the pons, cerebellum and medulla. Cells that form the boundaries of the rhombomere bulges proliferate much faster than those in the middle. It is very difficult for cells to cross from one rhombomere to another, so cells tend to stay within one rhombomere. Each rhombomere eventually gives rise to one or more vestibular neuron types. However, it is not necessarily dependent on segmentation. The motor nerves form depending on rhombomeric patterns, but each nerve can come from either one rhombomere or a pair of rhombomeres that are neighbors. Furthermore, the correct development of the various pharyngeal arches is believed to depend on interactions with specific rhombomeres. With these mechanisms, neural crest cells, for instance, from each rhombomere give rise to different ganglia, or clusters of neurons. Many of these rhombomeres have been mapped to an extent in species other than human. For example, r2 has been shown to give rise to the trigeminal ganglion, while r4 has been shown to give rise to the geniculate ganglion as well as spiral and Scarpa's ganglia. r5 and r6 gives rise to the abducens nerve, and the lower part of r6 and the upper part of r7 gives rise to the petrosal ganglion. Finally, the border of r7 that is not in contact with r6 gives rise to the jugular/nodose ganglia. These mappings, however, cannot be applied cross species. Studies have shown that the number of neurons in the hindbrain increase as a species evolves. For example, a mouse has more neurons than a slug, but a chimp has more neurons than a mouse. One can assume that part of this comes from having to adapt to an increase in the size of the species. Also, in some species, the rhombomeres are obviously segmented and stay this way for an extended period of time. In other species, the segmentation eventually disappears. For example, rhombomeres are detectable by light microscopy in Swiss/Webster strain of mice for up to embryonic day (E)10.5, but they disappear at E11.5. Many rhombomeres, if compared across species, do not form into the same things. For example, r2 and r3 have been mapped to the trigeminal motoneurons in many species. However, not all species have this correlation. Each rhombomere has its own set of receptors, and the same receptors can cause different actions. For example, one study showed that