Abstract
The typical picture of mongolism has been observed in both members of a proved monozygotic twin-pair, aged 15. There is no indication in the family history of any hereditary predisposition, and both, the pregnancy and the birth were quite normal. The author reviews all cases previously reported, and expresses the opinion that mongolism is due to some phenomenon occurring during the germinating period, that is to say during the initial stages of development of the egg.

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