Board Thread:Warhammer 40k General Discussion/@comment-5846164-20130301144956/@comment-3258762-20130302105423

The neophytes are given with two gene-seed organs, one smaller in the neck and one larger in the chest cavity. After an unspecified amount of time, the Gene-Seed matures and the one in the neck can be safely removed, while the chest one cannot be retrieved without killing the marine, hence why the Apothecaries recover it (and the neck gene-seed if not mature yet) from dead Battle Brothers.