Sewing Machines

Sex-differentiation.

It is now established that the sex of the individual is decided at the time of the union of ovum and spermatozoon, i.e., at the time of fertilisation; and the mechanism which determines sex has been disclosed. At the beginning of this century it was generally accepted that at the time of fertili sation the egg was sexually neutral and that the sex of the new individual was determined by the conditions incident to its de velopment. But certain facts are now known which point directly to the conclusion that sex in higher animals at least is prede termined at fertilisation. Identical twins are derived from a single fertilised ovum and are always of the same sex, whereas fraternal twins, originating in two distinct fertilised ova, may or may not be both of the same sex. There is no reason why, if purely environmental factors are at work in determining the sex of the offspring, those produced from one egg should always be of the same sex whereas offspring produced by separate ova may include both males and females.

In all higher bisexual animals so far as is known, and in certain plants, the sexes are to be distinguished by differences in the chromosomal content of the nuclei of the cells of which the body is built. (See CYTOLOGY.) In certain forms there is in one sex an unpaired chromosome in place of the equal pair in the case of the other sex. This difference is symbolised thus:—the sex with the unpaired chromosome is the XO ; the other the XX. In other forms the number of chromosomes is the same in both sexes, but while in one sex, the XX, the members of one pair are identical in size and shape, in the other, the XY, the single X has an unequal mate, the Y-chromosome. (In still other forms the

X-chromosome is represented not by one chromosome but by a group of from 2 to 8 which act together as a compound X-ele ment.) These chromosomes, the X and the Y, since in respect of them the sexes differ, are known as the sex-chromosomes.

Into each ripe germ-cell, ovum and spermatozoon, there passes one or other member of each pair of chromosomes. If, as is the case in man, the female is XX, all ova will be alike in that each will contain an X-chromosome, but there will be two kinds of sperm elaborated by the XY male, one carrying the X-chromo some, the other the Y. The female in these groups is monogametic, elaborating but one kind of gamete (marrying-cell) in respect of chromosome content, the male, digametic. When egg and sperm unite, into the fertilised egg there will pass, by way of the sperm, one member of each pair of chromosomes, and, by way of the egg, the other member of each pair. There will thus be two kinds of fertilised ova, one that received an X-chromosome from each parent, to become an XX individual, i.e., a female; the other, which received an X-chromosome from the mother and a Y from the father, to become an XY individual, i.e., a male. This chromosomal difference between the sexes provides a simple yet sufficient mechanism for sex-determination. Similar reasoning will apply to those species in which the X-chromosome in one sex has no mate, and also to those in which the X-element is com pound.

In mammals and in most insects the male has the single X; in the butterflies, moths and birds, on the other hand, it is the female that is digametic.