Note X — Chapter XI · DEVELOPMENT OF THE REPRODUCTIVE IMPULSE

Colonies arise when the individual animals do not leave each other, after their division, to live a separate life but remain together. After the second division there are four animals or members, after the third division eight, then sixteen, thirty-two, etc., until an entire colony of millions of cells has risen. The human body is composed of some twenty-six trillions of cells (26,000,000,000,000,000,000). Brain and spinal cord alone contain two billion cells.