There's an important phenomenon in development called neurulation. This is a process that starts with a flat sheet of ectodermal cells, folds them into a tube, and creates our dorsal nervous system. Here's a simple cross-section of the process in a salamander, but in general outline we humans do pretty much the same thing. Cells move up and inward, and then zipper together along the length of the animal to produce a closed tube.

It's a seemingly simple event with a great deal of underlying complexity. It requires coordinated changes in the shape of ectodermal cells to drive the changes in tissue shape, and invisible in simple diagrams to the right are all the inductive interactions going on that trigger the differentiation of the tube into a nervous system.

This is also a relatively early event in humans. It begins about 18 days after fertilization, with a thickening of the ectoderm to form the initial sheet, called the neural plate. By day 19, the edges of the plate thicken and rise, and the whole thing folds at the midline and looks something like an open hot dog bun. The photo below is of a pair of 20 day old human embryos; the midline seam is open and clearly visible.

By day 23, the edges have all fused together along the length of the embryo, leaving openings call neuropores at the rostral and caudal ends; the picture to the left is of a 23 day old human embryo, looking at its head end, and you can see right into its prospective brain. By days 26-28, both neuropores will close.

This is a crucial event that occurs very early, in the first month, of a pregnancy. What happens if the zipper gets stuck, and the tube fails to close completely?

It happens, and unfortunately it happens fairly frequently. About 0.1% of births in America have a serious neural tube defect caused by a stuck neural tube zipper.

The outcome is myelomeningocele—portions of the neural tube are left exposed, fail to develop properly, and are damaged and degenerate. It's variable in its consequences; when the posterior neuropore fails to close completely, it can result in nearly undetectable defects in the caudal spine to lower body paralysis and perturbations of cerebrospinal fluid flow that if untreated, can lead to hydrocephalus and severe brain damage.

Failure of the anterior neuropore to close is even more serious. The brain fails to form. This condition is called anencephaly, and it is untreatable and lethal. If they aren't dead at birth, they might last a few days before succumbing. They have no brain. At best, they have a mass of dying, relatively undifferentiated neural tissue smeared across the floor of their incompletely formed skulls. They can't think, they can't feel, they can't respond. The real tragedy is that development can proceed surprisingly far without a brain, and these fetuses are recognizably human (here is a photo for the strong of stomach), and they can be carried fully to term.

I can't imagine a clearer case to illustrate that humanity involves more than just the fusion of two gametes. We aren't defined by our complement of genes or a single instant of genetic combination, but are the result of many genetic and epigenetic processes working progressively through embryogenesis to assemble a functioning human being. When moral absolutists try to apply simple-minded, black-and-white reasoning to a complex situation (and defining a human being is certainly a complex problem), you get criminal travesties like this one:

A sailor's wife was pregnant with an anencephalic child, whose probability of surviving or of ever being conscious was zero. She, reasonably, wanted an abortion.

But the Congress had decided -- that no federal funds should be used to pay for abortions except where the life of the mother was at stake. As a result, Tricare (formerly CHAMPUS) the agency that covers military families, refused to pay the $3000 the abortion would cost.

The family sued, and a federal court ordered Tricare to pay, and the abortion went forward.

Then the Justice Department (with John Ashcroft as Attorney General) sued the family to recover the $3000, out of the sailor's pay of less than $20,000 a year.

The Justice Department just won. A panel of the Ninth Circuit ruled that, under a 1980 Supreme Court precedent upholding the Hyde Amendment -- a parallel provision to the one in question, but applying to Medicaid recipients rather than to military families -- the law was valid and the government didn't have to pay for the abortion. Consequently, the family has to pay the money back.

Our guardians of purity have magnified the pain of this family and willfully and vindictively punished them for the 'crime' of a biological imperfection. I call that evil, pure and simple. There should have been no question in this case that an abortion was necessary.

There are much more difficult cases. What if the fetus was diagnosed with 'merely' a case of myelomeningocele that meant it would be paralyzed, require extensive surgeries, and would be a crippling financial burden? The average cost per year of maintaining a child with myelomeningocele is approximately $70,000, and that drain never ends. People with severe spina bifida can be intellectually and socially capable, fully human, but a young family with limited resources ought to have the privilege of making a choice about whether to shoulder the responsibility before the fetus has acquired those mental capacities. I presume we now have a government that will force families to take on that burden, but will refuse to pay any part of the price.

Cohen MM (2002) Malformations of the craniofacial region: Evolutionary, embryonic, genetic, and clinical perspectives. American Journal of Medical Genetics 115(4):245-268.