The Developing Embryo: Introduction.
The Embryo - Introduction
The Germ Cells
Germ Line and Somatic Mutations
Conception and Development.
The SRY Gene.
Genetic Errors of Metabolism.
The Fetal Environment.
It is thought that early civilisations worshipped the mother as the bringer of life. There is evidence that, in later centuries, the belief became that the male became originator and the female was just a nurturing vessel. Perhaps it was this idea that fostered the taboo against masturbation.
Little, of course, was known about female internal anatomy, and nobody had seen an egg or a sperm. Aristotle is thought to be the first writer to suggest that both equally contributed, something to do with semen and menstrual flow, both being formed from "sanguineous nutriment". There was also the controversy between preformationism which suggested that in the male seed were all the characteristics of the new child, as a miniature being, and epigenesis which held that the complexity of development must be imposed from without by some vital force.*
For many years there were two opposing factions - the 'spermatists' and the 'ovists'. Fruitful material, it might be thought, for Jonathan Swift, when he wrote Gulliver's Travels in 1726, but it was the first serious challenge to the androcentric view of the world.
At the end of the seventeenth century, Van Loewenhoeck invented the microscope and, for the first time biologists became aware of bacteria and the single celled organisms, and within his own semen, he saw what he referred to as 'animalcules.' Within the sperm, some claimed to see what they thought was a complete preformed human being, the 'homunculus'.
Many of the discoveries in this area came from Germany. In 1827, von Baer described how all animals developed from ova, and in 1839, Barry observed a spermatozoan penetrating a mammalian egg. In 1855, Pringsheim showed it occurring in a freshwater alga.
Sex is a remarkably inconvenient way of reproduction. Arguments about why it evolved are a study in themselves. The example of the dandelion is often quoted, growing quite successfully without - although pollen is involved, it merely triggers development, and as with most plants in the daisy family, all the offspring are female.
It depends on two different pieces of genetic material, gametes, coming together by whatever means and fusing to form a zygote. One gamete, the sperm, consists of very little more than DNA. Being composed of such a small amount of material, it can be produced in vast quantities, and it is highly mobile. However, it has to make contact with its opposite number, an ova, or egg. Eggs usually contain the material and nourishment needed to produce the new organism. Hence they are relatively immobile and fewer in number. In biology, a male is defined as the producer of sperm, a female as the producer of eggs.
In humans, the female gamete is referred to as 'X'; the male gamete may be 'X' or 'Y'. Merged at the union, an 'XX' offspring will be female, an 'XY' offspring will be male.
However, errors occur. Sometimes offspring may be X0, XXY, XYY and so on. Moreover the ability of the genes within the zygote, or the resulting embryo, to produce a male or female individual may be blurred. Errors may occur in the complex process of development in the womb.
John Money(3) described seven criteria important for correct sexual development after conception.
Mind and Body
We need to be very clear at this point. An error in sexual dimorphism, somewhere in stages 1 to 5, gives rise to an intersex condition, which is a medical definition. Transsexualism is a definition in the psychological domain, referring to stage 7.
Another important point is that though the people concerned have been referred to, in the past, as pseudo-hermaphrodites, the preferred word is intersexed.
While the complete failure of certain physical processes of development seem unequivocal, the effects of hormones on the development of the brain are controversial. In particular, the debate implicates not only the embryo's own hormonal development, but the external environment of the mother, her womb and blood supply.
* Epigenesis now means something slightly different, as we described in the last section "Epigenesis"
Bibliography and Good Reading.
Bland, J., (1998-2004) About Gender: The Developing Embryo Introduction
Book graphics courtesy of Amazon.co.uk
Web page copyright Derby TV/TS Group. Text copyright Jed Bland.
08.04.98 Last amended 05.06.02, 19.03.03, 10.03.04, 23.10.04, 01.05.14