underdeveloped
adj
1 not fully grown or developed 未充分发育或发展的
underdeveloped muscles 不发达的肌肉.
2 (of a country, etc) not having achieved its potential in economic development (指国家等)经济不发达的.
is as the proportion of the total vocabulary of his mother tongue against those words of his own baby talk that are taken up into the vernacular of his family.