About 55 per cent of immature infants live until the end of the first year; 52 per cent reach the school age, and 51 per cent reach the age of puberty; in other words, the mortality after the school age is not any higher than for mature children. This is not true, unfortunately, of the physical as well as of the mental development of the immature infant.Ylpp\l=o"\, in his calculations, always referred to the conception age, i. e., to the intra-uterine and extra-uterine age of the child. He calculated the intra-uterine age from a fetal weight and length curve, so that when he speaks of a child 5 years of age, and that child was born at the end of seven months of pregnancy, the child's extra-uterine or citizen age is really 4 years and 5 months. I prefer to make my calculations on the basis of the extra-uterine age alone, because when a comparative study is made of the data with measurements of normal children, the latter are also based on the extra-uterine or official age alone.
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