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The mesencephalon or mid-brain, when looked at from the dorsal surface, shows a pair of large hollow swellings, the optic lobes or corpora bigemina.

The first is known as the cephalic, and is caused by the prosencephalon bending sharply downward, below and in front of the mesencephalon.

The single cavity seen in Amphioxus is here subdivided into three: an anterior or prosencephalon, a middle or mesencephalon, and a hinder or rhombencephalon.

In the mesencephalon fibres corresponding with those of the fillet of higher vertebrates can be seen, and there is a nucleus in the hinder part of the corpora bigemina foreshadowing the separation into corpora quadrigemina.

In the mesencephalon the alar laminae form the corpora quadrigemina; these at first are bigeminal and hollow as they are in the lower vertebrates.