Chick tail buds labeled with tritiated thymidine were transplanted homotopically to unlabeled embryos to determine the specific body levels and structures formed by tail buds. In addition, labeling patterns of posterior structures were studied in normal embryos labeled for progressively longer time periods.
Tail buds contributed labeled cells primarily to tail neural tube, somites, mesenchyme, and caudal arteries, but not to tail notochord, skin ectoderm, hindgut, or tail gut. Labeled somites were often located three to four somite levels anterior to labeled portions of neural tubes. In labeled normal embryos central notochordal cells were labeled only occasionally. This labeling pattern extended posteriorly into a region designated as prospective notochord, which merged posteriorly with the tail bud. The tail bud labeled uniformly and contained about the same density of labeled cells as the segmental plates, indicating that proliferation was not excessive in the former.
Defects in operated embryos occurred primarily in the neural tube, somites, and notochord. The tail portion of the notochord, presumably formed from the prospective notochordal region, was never isolated from the trunk portion. The tail portion of the neural tube rarely seemed isolated from the trunk portion. Large, unpaired midline somites frequently developed. They appeared to form only when a space was available in which paired segmental plates could approach the midline and fuse.