Early Localization of the Thyroid Anlage in Hyla regilla.

Abstract

Since the initial work of Vogt 1 on the localization of anlagen in the young amphibian gastrula, investigators have continued to map out on the anuran or urodele gastrula the presumptive areas which in normal development give rise to particular organs or structures. Although Holtfreter 2 has added considerably to the knowledge of these areas by explantation, the major number of the studies have utilized the technique of vital staining.

In a 3mm. tail-bud embryo of the Pacific tree frog, Hyla regilla, the thyroid is visible as a ventral outgrowth just anterior to the yolk mass which forms the floor of the archenteron. Röhlich 3 has shown that for Triton at least, the major part of the foregut arises from the dorsal lip of the blastopore. The location of the thyroid in the older embryo, however, gives no indication whether its origin is from the dorsal or the ventral lip.

Gastrulae were obtained in which the future crescentic blastopore was indicated only by heavier pigmentation or by a slightly puckered appearance of the cells. After removal of the jelly layers, but not the chorion, the embryos were placed in wax-bottomed watch glasses, with the blastopore uppermost. Bits of agar impregnated with Nile blue sulphate according to the method of Stone 4 were placed on the embryo and held firmly in place by a celluloid bridge. After half an hour the agar was removed and the embryo transferred to a dish of pond water. Two days later the animal was either dissected to locate the stain or was fixed by Yntema's method 5 for future sectioning. A total of 40 animals was thus studied, of which 27 were dissected, 8 were sectioned and stained, and 6 were discarded because the stain was too faint for accurate observation.