[The population aspects of thyroid development in the Siberian (Lemmus sibiricus) and Arctic (Dicrostonyx torquatus) lemmings of Wrangel Island].

Rate of thyroid gland development differ significantly on the stages of growth, highest value and recession of quantity in Lemmus sibiricus fetuses. Rate of embryogenic thyroid gland development in the highest value of population quantity reduces compared to that observed during the quantity growth. However, this reduction is displayed in smaller extent than in embryos of Dicrostonyx torquatus of the same population cycle stage. In newborns organ achieves definitive structure in highest value, which differs in 2-3 days from the terms of definitive structure forming in Dyscrostonyx torquatus. On the stage of population quantity recession thyroid gland differentiation speed increases and definitive structure sets in by 17th prenatal day. Comparison of differentiation terms of Lemmus sibiricus and Dicrostonyx torquatus thyroid gland on the stages of growth, highest value and recession of population quantity indicates the presence of principal resemblance between them.