Bilateral Localization of Prospective Thyroid in the Early Chick Blastoderm,Studied in Chorio-Allantoic Grafts.

The vascularized chorio-allantoic membrane of the chick is considered a practically neutral medium in which potencies of embryonic areas may be tested. This technique of chick embryology has been described by Willier. 1

Histological examination of grafts of the whole pellucid area of blastoderms of the head-process stage, allowed to remain 8 or 9 days on the host membrane, shows normal or almost normal differentiation of a great many tissues: among these thyroid. 2 Embryonic organization at this stage is better revealed in grafts of definite isolated parts of the blastoderm. The study∗ to be reported has been made on such transplants with regard to the thyroid.

Two types of graft from the head-process stage have been analyzed with reference to differentiation of the thyroid. In the first series, the area pellucida anterior to the level of the primitive node was divided by a median cut into right and left halves: these pieces were grafted separately. In the second series, the same area was divided longitudinally into 3 parts. An attempt was made to keep the width of the median piece that of the node, or slightly greater. In this series, left, median, and right pieces were transplanted separately.

Series 1. In this series, 5 pairs of grafts, each pair consisting of a right and a left half originating from a single donor, have been examined. Of the 5 left halves, each one contains from one to 3 masses of thyroid tissue. The right halves show less uniformity in this respect. Of the 5, three unquestionably contain thyroid. There is one doubtful case; and one negative. The fact that in 3 indubitable cases out of 5 both grafts of a pair contain thyroid, is conclusive proof that there is prospective thyroid material in both halves of the blastoderm at the head-process stage.