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that queen bees produced male drones through parthenogenesis and female offspring
from eggs fertilized by sperm. Contrary to Owen’s definition, it was clear that new organisms could develop solely from eggs. Siebold also uncovered that not only can eggs develop into fully
formed animals quite without any fertilization event but also that parthenogenesis was by no means an exceptional occurrence, something peculiar to aphids. He made a point of countering the idea
that ‘development of the eggs can only take place under the influence of the male semen’. This age-old concept, he wrote, ‘has suffered an unexpected blow’. Rather than
being a result of some undefined force of questionable existence, parthenogenesis was an independent, fixed, orderly event.
    But if eggs could develop on their own, as Siebold had proved, then what was the point of the male? Based on Siebold’s work, Darwin made a remarkable conjecture: ‘I have often
speculated for amusement on the subject, but quite fruitlessly,’ he wrote to his friend Huxley, ‘But the other day I came to the conclusion that some day we shall have cases of young
being produced from spermatozoa ... without [an egg].’ Darwin had a point: if eggs could independently generate life, why couldn’t sperm do it, too? And if not little people, what was
inside the sperm, and how were these seemingly living creatures made?

    In 1905, Jacques Loeb provided an answer. Loeb, a physiologist working in Germany, was busy trying to force unfertilized eggs to develop into embryos.
Using alkaline or acid solutions, potassium, and salt, even ox blood and cane sugar, he triggered development in the unfertilized eggs of sea urchin, starfish, marine molluscs, and other creatures.
For the first time in history, someone had managed to create new life in the laboratory with
no sperm at all
.
    In working out what to substitute for sperm, Loeb realized he needed to find something that must have two effects on the egg. ‘In the first place’, he wrote, to ‘cause... its
development’ and in the second to ‘transmit... the paternal characters to the developing embryo’. For the marine species with which he was experimenting, the ability to cause the
embryo to develop was enough. Baby sea urchins born in his lab would need no fathers from which to acquire paternal characteristics. The same could not be said if the subject were not sea urchins
but humans.
    The fluid praised as the essence of life by Aristotle and Galen (and the innumerable others who came before and after) is indeed remarkable. Human semen is a rich cocktail, a combination of
sugars, salts, enzymes, vitamins, and minerals, including such truly essential ingredients as fructose, sorbitol, inositol, phosphorus, zinc, magnesium, calcium, potassium, ascorbic acid (vitamin
C), and cobalamin (vitamin B12). As the ancient thinkers suspected, it is also the medium through which a father provides his set of instructions for making offspring. But unknown to these early
natural philosophers, some part of semen – about five percent of what a man ejaculates – contains fifty million to two hundred million sperm. These cells are highly specialized, built
to travel up to four millimetres a minute and to release chemicals that can target and penetrate the egg.
    The creation of sperm begins inside the testes of a pubescent boy, when the solid cords that had transected these glandsthroughout his childhood begin opening up into
tubes. The process carves a space at the cord’s centre through which fluids will eventually be able to pass. These tubes will become contorted and so numerous and fine that in an adult male
testicle, their collective length will measure as much as 350 metres, or more than one thousand feet. They will also become home to the stem cells that become sperm, called
spermatogonial stem
cells
, or SSC. Stem cells are by definition immature, in that they are somewhat undecided as to their identity and