Some points of early language development were clarified through the survey for the vocabulary checklist (the category of animals's names, toys, vehicles and food and drink) of the Japanese version of MacArthur Commuincative Development Inventories for children from the age of 1O months to 36 monhts. Vocabulary size increased with the age. Girls' vocabularys were larger than boys', and significant differences occurred at the age of 24 months. The children begin to produce adult words at the age of 15 months.
The transition points of baby word to adult word occurred around 21 months.
The generalization of a word occurred at the age of 12 months. The peak age of the generalization was at 21 months. After that, differentiation and specification of words were established.
These transitions in the early language development occur in relation to mothers' addressed language to childre and children's cognitive growth.