The thyroid gland first appears in the phylogenic scale in the Lamprey larva, Ammocoetes, at the time of metamorphosis (see review by Constantinescu, 1972). In higher Vertebrates the adult thyroid gland consists of vesicles i. e. thyroid follicles containing colloid and lined with a cubic or prismatic epithelium. Since the end of the 19th century, many authors have studied the morphoge­ nesis of the follicles during the embryonic and fetal development of the gland in Man and other species, principally Chick, Rat and Rabbit. The development of techniques for culturing organs of higher animals, in particular the thyroid by Carrel and Burrows (1910) and Champy (1914,