. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . J*^* .9^ Fig. I. Thyroid from a 9 days old embryo. The gland is of a cordonal type and cells are practically devoid of canalicu- lar structures and colloid. Mitochondria and small vacuoles are visible. Magnification 5000. and chicks from 8 days of incubation to 13 days after hatching (Light Sussex). Thyroids were also removed from 18 embryos of 9 and 16 days previously treated at 3 days with thyroxine ( mg) or tetramethylthiourea ( mg). Choice and dosage of the drugs were established from our previous inv
. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . J*^* .9^ Fig. I. Thyroid from a 9 days old embryo. The gland is of a cordonal type and cells are practically devoid of canalicu- lar structures and colloid. Mitochondria and small vacuoles are visible. Magnification 5000. and chicks from 8 days of incubation to 13 days after hatching (Light Sussex). Thyroids were also removed from 18 embryos of 9 and 16 days previously treated at 3 days with thyroxine ( mg) or tetramethylthiourea ( mg). Choice and dosage of the drugs were established from our previous investigations. The glands were fixed in osmic acid, buffered according to Palade (1952) and embedded in n-butyl methacrylate. Sections /< in thickness were cut with a Porter-Blum microtome and examined in a Philips electron microscope (Mod. 1951). In the diflferentiation of the thyroid, electron microscope shows two periods identical to those previously described. In the first one, before 12 days of incubation, the canalicular structures and the col- loid are very poor (fig. 1). At 8 days, the gland consists of cellular cords which form spherules progressively later. Cellular nuclei contain a nucleolus and are ovoid, sometimes bean-shaped. The cytoplasm possesses a clearly dis- cernible chondriome and numerous vacuoles variable in size and containing a pale substance. The cana- licular structures, very scarce and small, lie generally at the periphery of the cell and show a clear lengthened center surrounded by a thii osmophilic wall. Be- tween 2 or 3 epithelial cells, some small colloid droplets appear, containing filamentous protrusions, or microvilli inserted on the cellular membrane. The second period is characterized by the differ- entiation of follicular colloid associated with sudden and general distribution of the canalicular structure. Its beginning is already discernible in the thyroid of 12 days old embryo. Sometimes the cell, very poor in canaliculi, presents t
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