Vestigeal (L. vestigium, footstep), representing organs which existed once in a more developed condition.
Volvox (L. volvo, roll), a small fresh-water organism occurring in spherical colonies.
Wasserman Reaction, a test for syphilis, see [p. 188].
X-Element, same as sex-chromosome.
Zygote (Gr. zygon, yolk), the product of the union of two gametes.