20 The Mechanistic Conception of Life 



fertilized it is always done by a spermatozoon which contains 

 an X-chromosome. The egg has, therefore, after fertilization 

 in these animals always two X-chromosomes and from such 

 eggs only females can arise. 



It had been known for a long time that in bees and ants 

 the unfertilized eggs can also develop, but such eggs give 

 rise to males only. This is due to the fact that the eggs of 

 these animals contain only one X-chromosome and from eggs 

 with only one chromosome only males can arise (at least in the 

 case of animals in which the male is heterozygous for sex). 



The problem of sex determination has, therefore, found a 

 simple solution, and simultaneously Mendel's law of segrega- 

 tion also finds its solution. 



In many insects and in man the cells of the female have 

 two sex chromosomes. In a certain stage of the history of the 

 egg one-half of the chromosomes leave the egg (in the form 

 of the '^ polar-body") and it keeps only half the number of 

 chromosomes. Each egg, therefore, retains only one X or 

 sex chromosome. In the male the cells have from the begin- 

 ning only one X-chromosome and each primordial sperma- 

 tozoon divides into two new (in reality into two pairs of) 

 spermatozoa, one of which contains an X-chromosome 

 while the other is without such a chromosome. What can be 

 observed here directly in the male animal takes place in 

 every hybrid; during the critical, so-called maturation divi- 

 sion of the sexual cell in the hybrid, a division of the chromo- 

 somes occurs, whereby only one-half of the sex-cells receive 

 the hereditary substance in regard to which the two original 

 pure forms differ. 



That this is not a mere assumption can be sho"v\Ti in those 

 cases in which the hereditary character appears only, or pre- 

 eminently, in one sex as, e.g., color blindness which appears 

 mostly in the male. If a color-blind individual is mated with 

 an individual with normal color vision the heredity of color 



