IT was long believed that the sex of the embryo is determined at a relatively late stage in its development, and therefore it seemed probable that external factors must decide whether the embryo is to become a male or a female individual. Many views have been held as to what these external factors are, and from time to time hopes have been held out that it might be possible to regulate, by artificial means, the sex of the developing embryo.
In the last few years opinion has begun to turn in the opposite direction, and several attempts have been made to prove that the sex of the embryo is determined in the egg. That this must be the case in man seemed to be indicated by the fact that 'identical twins' are always of the same sex. There can be little doubt that such twins come from the same egg, and the presumption is strongly in favor of the view that they represent the separated first two cells of the segmenting egg. These twin embryos are enclosed in the same chorion, which further indicates that they have come from one egg. The 'ordinary twins' of man are no more like each other than are any other two children born at different times. The pair of ordinary twins often consists of a male and a female. Since the embryos that give rise to ordinary twins are subjected to practically the same conditions during their uterine life, and are often, as has been said, a male and a female in a pair, it follows that in man the external conditions that affect the egg, after it has left the ovary or after it has been fertilized, do not determine the sex. A similar and even more remarkable fact is known