Eggs are the female gametes.  Gametes carry the chromosomes from one generation to the next.  Both male and female gametes contribute chromosomes to the offspring; in the case of humans, both male and female gametes contribute 23 chromsomes each.  Each fertilised egg therefore contains 46 chromosomes.  This fertilised egg divides repeatedly by a special type of cell division (mitosis) to give a ball of cells, which will go on to form an embryo.

Generally speaking, for most species, many more male gametes are produced than female gametes, they are also much smaller and can move - they swim towards the female gamete ( the egg). They are,therefore, "many, minute and motile".  Eggs, the female gametes, are much larger, they cannot move (of their own accord) and are not produced in such large numbers - they are, therefore, "few, fat and fixed".

Eggs or ova are produced in structures called ovaries.  In human ovaries, there are many immature eggs but only a few develop fully.  Human eggs are about 100 micrometres in diameter, which is quite large for an animal cell. Generally speaking, an egg is released once a month from the ovary and is wafted into the funnel of the oviduct, and then moves down the fallopian tube or oviduct - due to the ciliated epithelium of the latter. The oviduct is lined with tiny hair-like structures that help move the egg).  It is in the upper part of the oviduct that egg and sperm meet, and where fertilisation occurs.  The fertilised egg then continues its journey down the oviduct to the uterus, where it implants and dividing again and again.

In relatively recent times, fertilisation of human eggs has been achieved 'in vitro' - literally "in glass"; a technique that was to become inportant for some infertile couples.  This was first done at Cambridge by Dr Robert Edwards.  At about the same time, Dr P Steptoe 'harvested' human eggs laparoscopically ( using keyhole surgery).  The two began to work together and in 1975, they attempted the implantation of a fertilised egg in an infertile volunteer; unfortunately this resulted in an ectopic pregnancy (where the embryo develops outside the uterus - sometimes in the fallopian tube; this is dangerous to the mother and generally needs surgical intervention).  In 1977, they collected an egg from the ovary of Lesley Brown, fertilised with sperm collected from her husband and then implanted the egg back into Lesley.  In July, 1978 Louise Brown was born - the first IVF (in vitro fertilisation) baby.

Comments on this article

Jack Horder sucks eggs 4 March, 2014

Very informative.

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