The tetrocerebral ganglonic centres are located between the tritocereb
ral and mandibular ones. The evolution of the tetrocerebron is charact
erized by its movement to the anterior most centres, so that it comes
to join side by side with the tritocerebron. However, the tritocerebro
n and the tetrocerebron remain clearly distinc from one another. The e
volution of the tetrocerebral commissure follows nearly parallel pathw
ays.