ABSTRACT
The pattern of an annual layer is assessed first of all by the contrast of its principal element, and secondary by the presence or absence of additional elements. The first annual layer in many mammals is characterized by indistinct principal element. The role of selection, or of a factor synchronizing animal growth rhythms, can be played not only by seasonal changes of climate as such but also by different seasonal changes of living conditions, for instance by seasonal changes in the availability of food, etc. Laboratory animals and those domestic animals that are kept indoors all year round are in different conditions. In pet dogs, cementum growth layers were much less distinct than in sledge dogs. Growth layers in cementum were described in a number of tropical ungulates. It was established that in some cases two incremental lines and in others one incremental line was formed annually.
