The Mechanistic Conception of Life 19 



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 being 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 w^hether a boy or a girl originates; and 

 it agrees with the law of probability that in a large popula- 

 tion the number of boys and girls born within a j'ear is 

 approximately the same.^ 



These discoveries solved also a series of other difficulties. 

 Certain types of twins originate from one egg after fertiliza- 

 tion. 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 fertilization. It was known that from fertilized 

 eggs in these animals only females develop, males never. 

 It was found that in these animals the eggs contain only one 

 sex chromosome; while in the male are found two kinds of 

 spermatozoa, one with and one without a sex chromosome. 

 For Phylloxera and Aphides it has been proved with certainty 

 by Morgan and others that the spermatozoa which contain no 

 sex chromosome cannot live, and the same is probably true 

 for bees and ants. If, therefore, in these animals an egg is 



1 It is stated that the number of males born exceeds that of the females by a 

 slight percentage. If this statement is correct it must be due to a secondary 

 cause, e.g., a greater motility or greater duration of life of the male spermatozoon. 

 Further researches will be needed to clear up this point. 



