To all appearances, that inscrutable simplest of living things, the fertilized ovum, beginning of the human, starts bisexual, double sexed, both masculine and feminine, or perhaps neither masculine nor feminine. Then a form develops. Then within that form a patch of cells arise which the microscopist recognizes as the forerunners of the male or the female reproductive cells. Then some more development. And at birth, sex is definitely settled, as far as the reproductive organs are concerned. Our knowledge here, as everywhere, is still fragmentary. Statistical reviews seem to show that in times of stress, war, famine, pestilence, more boys are born than girls. But that is neither here nor there. It sheds no further light on the subject. Monosexuality is a distinction of the human species: the sexes are pretty clearly differentiated. In some animals, such as some worms, there is a bisexuality of the individual. There are present the reproductive organs of both sexes, capable of impregnating other individuals as well as of being impregnated. In some of these, even self-impregnation may occur. This is the condition of hermaphroditism. But the higher up one goes in the scale of evolution, the greater becomes the distinction between the sexes. Anatomic hermaphroditism becomes a rare anomaly. Life appears to have perfected this trick of separate sexes, sex specialization, in short, for the sake of the efficiency which goes with specialization. When a germ cell divides, its nuclear material breaks up into segments known as chromosomes. Now it has been found, for example in the case of the common squash bug, anasa tristis, that there are 22 chromosomes in the female, and 21 in the male. In the female two of these are visibly different from the rest, while in the male there is one odd one, the remaining 20 being like the corresponding 20 of the female. Before the germ cell becomes fit to mix with a germ cell of opposite sex, in the process of fertilization, it must lose one half of these. So the number of chromosomes for the species is kept the same or constant. This is the process of maturation. In the process, when the chromosome number is halved among the females, 11 go into each mature egg. But among the males, the odd chromosome, also known as the X-chromosome, can perforce go only into half of the sperm cells, leaving the others without it. So the sperm are formed in equal numbers of 10 and 11 chromosomes respectively.