lodohormones in the serum of chick embryos and post-hatching chickens as influenced by incubation temperature. Relationship with the hatching process and thermogenesis

temperatures in a forced-draught laboratory incubator. A low incubation temperature resulted in a longer incubation period, whereas eggs incubated at a higher temperature hatched sooner. In all the temperature groups, serum T3 and T4 levels increased during the incubation period studied. Whatever the total duration of incubation within the experimental conditions, maximal serum T3 and Tg levels were always obtained the day of pipping. Embryos having perforated the air-space membrane the day before pipping showed elevated serum T3, but not T4, levels as compared to embryos without perforation. The presence of high serum T3 levels in chick embryos after perforation of the air-space membrane, and the sharp increase in the T3/, ratio before pipping were indicative of the important role of T 3 in the processes of pipping and hatching. After days 16 to 17, depending on the incubation temperature, a plateau for heat production (measured by indirect calorimetry) was reached while serum T3 and T4 levels were still increasing. Following the event of pipping, there was a rapid increase in heat production. A plateau might be due to the physical impossibility of each embryo to react upon an increase in T3 and T4 secretion by an increment in oxygen consumption, and would not exclude a relation between iodohormone levels and thermogenesis during development.