14 THE POPULAR SCIENCE MONTHLY 



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, espe- 

 cially 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 {Alraxas). In these animals the sex is predeter- 

 mined 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 the egs will be fertilized by spermatozoa with and one half by sper- 

 matozoa 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 fe- 

 males — 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-chromo- 

 some 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 



