eLife Assessment
This study is useful and unique, since hagfish brains are of phylogenetic importance and can reveal features ancestral to all vertebrates. The manuscript is, however, incomplete and would benefit from contextualization with the current literature; comparisons with the recent amphioxus study are suggested, plus an increased focus on the specific, unique features of the hagfish brain. One significant concern is the apparent absence of Datx2 expression, given that the riboprobe was synthesized from cDNA derived from whole-brain RNA extracts. Ideally, the authors should identify a tissue in which Datx2 is known to be strongly expressed and then apply the probe as a positive control.