protovertebra

from The Century Dictionary.

• noun In Carus's nomenclature (1828), a rib regarded as a vertebral element developed to contain and protect the viscera, or organs of vegetative life: correlated with deutovertebra and tritovertebra.
• noun A primitive, temporary vertebra; one of the series of segments which appear in pairs in the early embryo along the course of the notochord, and from or about which the permanent vertebræ are developed. They soon disappear, being replaced by definitive vertebræ.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy 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.