Page:Popular Science Monthly Volume 80.djvu/19

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15
THE MECHANISTIC CONCEPTION OF LIFE

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 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, therefore, 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 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 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, therefore, 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) 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 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 corresponds 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--