The Collaborative International Dictionary of English v.0.48:

Protovertebra \Pro`to*ver"te*bra\, n.; pl. Protovertebr[ae] .
   [Proto- + vertebra.] (Anat.)
   One of the primitive masses, or segments, into which the
   mesoblast of the vertebrate embryo breaks up on either side
   of the anterior part of the notochord; a mesoblastic, or
   protovertebral, somite. See Illust. of Ectoderm.
   [1913 Webster]

   Note: The protovertebr[ae] were long regarded as rudiments of
         the permanent vertebr[ae], but they are now known to
         give rise to the dorsal muscles and other structures as
         well as the vertebral column. See Myotome.
         [1913 Webster]