A SHORT WHILE ago, in mid-1978, the newest astonishment in medicine, covering all the front pages, was the birth of an English baby nine months after conception in a dish.

The older surprise, which should still be fazing us all, is that a solitary sperm and a single egg can fuse and become a human being under any circumstance, and that, however implanted, a mere cluster of the progeny of this fused cell affixed to the uterine wall will grow and differentiate into eight pounds of baby.

This has been going on under our eyes for so long that we've gotten used to it; hence the outcries of amazement at this really minor technical modification of the general procedure - nothing much, really, beyond relocating the beginning of the process from the Fallopian tube to a plastic container and, perhaps worth mentioning, the exclusion of the father from any role likely to add, with any justification, to his vanity.

There is, of course, talk now about extending the technology beyond the act of conception itself, and predictions are being made that the whole process of embryonic development, all nine months of it, will ultimately be conducted in elaborate plastic flasks. When this happens, as perhaps it will someday, it will be another surprise, with more headlines.

Everyone will say how marvelously terrifying is the new power of science, and arguments over whether science should be stopped in its tracks will preoccupy senatorial subcommittees, with more headlines. Meanwhile, the sheer incredability of the process itself, whether it occurs in the uterus or in some sort of vitro, will probably be overlooked as much as it is today.

For the real amazement, if you want to be amazed, is the process. You start out as a single cell derived from the coupling of a sperm and an egg, this divides into two, then four, then eight and so on, and at a certain stage there mereges a single cell which will have as all its progeny the human brain. The mere existence of that cell should be one of the great astonishments of the earth.

People ought to be walking around all day, all through their waking hours, calling to each other in endless wonderment, talking of nothing except that cell. It is an unbelievable thing, and yet there it is, popping neatly into its place amid the jumbled cells of every one of the several billion human embryos around the planet, just as if it were the easiest thing in the world to do.

If you like being surprised, there's the source;. One cell is switched on to become the whole trillion-cell, massive apparatus for thinking and imagining and, for that matter, being surprised. All the information needed for learning to read and write, playing the piano, arguing before senatorial subcommittes, walking across a street through traffic or the marvelous human act of putting out one hand and leaning against a tree, is contained in that first cell. All of grammar, all syntax, all arithmetic, all music.

It is not known how the switching-on occurs. At the very beginning of an embryo, when it is still nothing more than a cluster of cells, all of this information and much more is latent inside every cell in the cluster. When the stem cell for the brain emerges, it could be that the special quality of brainness is simply switched on. But it dould as well be that everything else, every other potential property, is switched off, so that this most specialized of all cells no longer has its precursors' option of being a thyroid or a liver or whatever, only a brain.

No one has the ghost of an idea how this works, and nothing else in life can ever be so puzzling.; If anyone does succeed in explaining it, within my lifetime, I will charter a skywriting airplane, maybe a whole fleet of them, and send them aloft to write one great exclamation point after another, around the whole sky, until all my money runs out. CAPTION: Pictures 1 and 2, A human egg at four-cell and eight-cell stages