DEVELOPMENT OF THE HUMAN BEING 
We may carry this remarkable consideration still fur- 
ther. It is difficult, though not always impossible, to 
detect differences microscopically between the germ-cells 
of man and those of many other creatures of the millions 
of species representing vertebrate and invertebrate life. 
These others also have each one their millions of ancestral, 
living, or potential individuals. By so much the more 
extraordinary, therefore, is the certainty of determination 
which stamps uniquely the order, species, race, family, 
sex, and individuality upon a microscopic human germ- 
call 
How is this possible? It is because of the astonishing 
divisibility of matter. Though so minute, the germ-cells 
are nevertheless large enough to contain at least billions, 
perhaps even trillions, of molecules apiece. In a structure 
containing such an Grichinieable number of molecules, the 
possibilities of dissimilar combinations of chemical differ- 
ences and of varieties of arrangement are sufficient even to 
carry all the complexities of inheritance which are wrapped 
up within a germ-cell. 
Let us look more closely upon this mystery. The whole 
substance of every living creature, plant and animal, is 
made up of minute cells. In the adult human body, the 
cells are estimated to number twenty-six quadrillions. 
Each of them contains a microscopic portion called the 
nucleus. This latter is the part in which inheritance fac- 
tors reside. Differences exist in the cells which determine 
if a fragment of substance is perchance part of a plant, an 
invertebrate, a mammal, a male or female, a brain, a 
nerve, a muscle, or skin. Living cells have four properties: 
1. Movement. 
2. Metabolism, nutrition, etc. 
3. Sensitivity. 
4. Reproduction. 
Movement, by expansion and contraction; metabolism, 
including the building up of definite substances and the 
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