THE MECHANISTIC CONCEPTION OF LIFE 15 



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-chromo- 

 some can not live, and the same is probably true for bees and ants. If, 

 therefore, in these animals an egg is fertilized it is always done by a 

 spermatozoon which contains an X-chromosome. The Qgg has, there- 

 fore, after fertilization in these animals always two Z-chromosomes and 

 from such eggs only females can arise. 



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

 fertilized 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 

 JT-chromosome and from eggs with only one chromosome only males 

 can arise (at least in the case of animals in which the male is heterozy- 

 gous for sex). 



The problem of sex determination has, therefore, found a simple so- 

 lution, and simultaneously Mendel's law of segregation finds also 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 leaves the egg (in the form of the " polar-body ") and 

 the egg keeps only half the number of chromosomes. Each egg, there- 

 fore, retains only one X or sex-chromosome. In the male the cells have 

 from the beginning only one X-chromosome and each primordial 

 spermatozoon divides into two new (in reality into two pairs of) sper- 

 matozoa, one of which contains an X-ehromosome 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 division of the sexual cell in the hybrid a division of 

 the chromosomes 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 shown in those cases in 

 which the hereditary character appears only, or preeminently, 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 blindness in the next two generations corre- 

 sponds quantitatively with what we must expect on the assumption that 

 the chemical substances determining color vision are contained in the 

 sex-chromosomes. In the color-blind individual something is lacking 

 which can be found in the individual with normal color perception. 

 The factor for color vision is obviously transmitted through the sex- 



