According to McClung each animal forms two kinds of spermatozoa in equal numbers, which differ by one chromosome. One kind of spermatozoa produces male animals, the other female animals. The eggs are all equal in these animals. More recent investigations, especially by E. B. Wilson, have shown that this view is correct for many animals.
While in many animals there are two kinds of spermatozoa and only one kind of eggs, in other animals two kinds of eggs and only one kind of spermatozoa are formed, e. g., sea-urchins and certain species of birds and of butterflies (Abraxas). In these animals the sex is predetermined in the egg and not in the spermatozoon. It is of interest that, according to Guyer, in the human being two kinds of spermatozoa exist and only one kind of eggs; in man, therefore, sex is determined by the spermatozoon.
How is sex determination accomplished? Let us take the case which according to Wilson is true for many insects and according to Guyer for human beings, namely, that there are two kinds of spermatozoa and one kind of egg. According to Wilson all unfertilized eggs contain in this case one so-called sex chromosome, the X-chromosome. There are two kinds of spermatozoa, one with and one without an X-chromosome. Given a sufficiently large number of eggs and of spermatozoa, one half of thewill be fertilized by spermatozoa with and one half by spermatozoa without an X-chromosome. Hence one half of the eggs will contain after fertilization two X-chromosomes each and one half only one X-chromosome each. The eggs containing only one X-chromosome give rise to males, those containing two X-chromosomes give rise to females—as Wilson and others have proved. This seems to be a general law for those cases in which there are two kinds of spermatozoa and one kind of eggs.
These observations show why it is impossible to influence the sex of a developing embryo by external influences. If, for example, in the human a spermatozoon without an X-chromosome enters into an egg, the egg will give rise to a boy, but if a spermatozoon with an X-chromosome gets into the egg the latter will give rise to a girl. Since always both kinds of spermatozoa are given off by the male it is a mere matter of chance whether a boy or a girl originates; and it agrees with the law of probability that in a large population the number of boys and girls borne within a year is approximately the same.
These discoveries solved also a series of other difficulties. Certain types of twins originate from one egg after fertilization. Such twins have always the same sex, as we should expect since the cells of both twins have the same number of X-chromosomes,
In plant lice, bees and ants, the eggs may develop with and without