by ANNA M. HERRERA, SIMONE G. SHUSTER, CLAIRE L. PERRITON, and MARTIN J. COHN
Highlights
During bird evolution, the phallus was reduced or lost in most lineages
Chickens form a reduced phallus, but primitive outgrowth mechanisms are conserved
A derived Bmp4 domain causes apoptosis and regression of the embryonic chick phallus
Basal birds and ducks lack distal Bmp4 expression and develop elongated phalluses
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