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

Serum levels of triiodothyronine (T 3 ) and thyroxine (T 4 ) were measured by RIA in developing chick embryos of the Rhode Island Red strain incubated at different 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 T 3 and T 4 levels increased during the incubation period studied. Whatever the total duration of incubation within the experimental conditions, maximal serum T 3 and Tg levels were always obtained the day of pipp- ing. Embryos having perforated the air-space membrane the day before pipping showed elevated serum T 3 , but not T 4 , levels as compared to embryos without perforation. The presence of high serum T 3 levels in chick embryos after perforation of the air-space mem- brane, and the sharp increase in the T 3/ T, ratio before pipping were indicative of the impor- tant 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 calori- metry) was reached while serum T 3 and T 4 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 T 3 and T 4 secretion by an increment in oxygen consumption, and would not exclude a relation between iodo- hormone levels and thermogenesis during development.