The Araucana rumpless chicken mutant. Adult skeletons showing the pelvis, synsacrum and caudal vertebrae. (A) In the homozygous mutant no caudal vertebrae develop (circle). The synsacrum develops normally and the bony plate over the fused vertebrae forms as expected. There is a hole in the final vertebra leaving the neural tube exposed. (B) In the heterozygote, 2 to 4 fused elements form beyond the synsacrum, in place of the free caudal vertebrae.