MAN FROM THE FARTHEST PAST 
cells which occurs thereafter throughout the body of the 
child, and even in adult life, the daughter cells, being 
always composed of halves of all chromosomes in ae pre- 
existing parent cells, contain equal shares of male and 
female elements. Every part of the human body, there- 
fore, is composed of cells which owe half of their chromo- 
somes to each parent. 
There is a reservation to be made. In the human species 
the male germ-cells, or spermatozoa, consist of about 
equal numbers of two kinds. Each of these germ-cells has 
one chromosome possessing one or the other of two dis- 
similar properties. Depending on which of these dissimilar 
chromosomes is included in the male germ-cell which 
fortuitously unites with the female, the resulting embryo 
is male or female. In some animals, as among the birds, 
for instance, the female holds the pair of unequal chromo- 
somes and is the governing influence which controls the sex 
of the offspring. In certain animals the disparity between 
the pair of unequal chromosomes is so great that one 
chromosome is entirely absent in half the male germ-cells 
and all cells of the bodies of females contain one more 
chromosome than those of males. 
After the union of the male and female germ-cells the 
resulting new cell of compound nature soon divides in the 
manner described above, making two cells, and these in 
turn divide again, and so on, until soon the ovum contains 
not one but many cells, each of which includes the male 
and female chromosomes. Up to this point, so far as 
microscopic observation shows, the cells have been nearly 
alike, merely minute sparks of living matter. Yet it 1s not 
quite so, for the descendants of certain of them have, it 
is now proved, capacity for only one sort of further de- 
velopment, although, on the other hand, other cells can 
subsequently give rise to any organs indiscriminately. 
But now, as in colonies of bees or of ants, the cells begin 
to be definitely assigned to different functions, and their 
descendent cells develop differences from this time for- 
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