When does an embryo become a human being? Britain's decision may set precedent for others

When does a human embryo become a human being? This is the essential - and still unresolved - question that has confronted test tube baby researchers for over a decade. Until it is answered, there is no biologically rational basis on which to resolve the subsidary ethical question: At what point is the developing mass of cells that results from a fertilized egg no longer a suitable subject for research?

Nevertheless, after deliberating for two years, Britain's Committee of Inquiry into Human Fertilization and Embryology has somewhat arbitrarily placed the boundary between impersonal cell mass and human being at 14 days' development. This probably is the most important aspect of the committee's recent report, although it does make many other substantive recommendations. For example, it urges a ban on use of surrogate mothers to bring other women's embryos to term. And it would place all research on human embryos under the supervision of a licensing authority.

The recommendations of this government-mandated study are likely to be written into law. This would be the first time that the point at which an embryo becomes ''human'' has been legally defined. It could set a precedent for other nations. The committee report has arrived in time for hearings on the implications of test tube baby technology held yesterday and today by a science and technology subcommittee of the US Congress.

Unfortunately, as the British committee's chairman, Dame Mary Warnock, has pointed out, this does not settle the question biologically. It does not satisfy those who agree with the minority of three committee members who hold that humanness is conferred at conception and so no embryo research should be allowed at all. Conversely, some embryologists consider 14 days too early a stage to call an embryo ''human.''

At that stage, the so-called primitive streak - the first identifiable embryonic feature - develops. Thus a number of European countries, Japan, and the US have adopted 14-day rules in practice if not yet in law. But some scientists suggest that humanness begins only with the early signs of brain development - a brain-birth criterion. Thus, in Denmark, the specified limit is 21 days.

According to news reports, Dame Mary says her committee opted for 14 days because ''we wanted a balance between very strongly held beliefs for total protection of the embryo and the strong desire to promote research.'' The committee wants the 14-day limit written into law, she explains, because ''all the things people dread,... like producing a baby entirely outside the womb, are too important to allow responsibility to rest with the licensing authority.'' She adds, ''We want 14 days to be the absolute limit.''

Thus an ''absolute limit'' on embryo research - which may well become a legal definition of embryonic humanness - is proposed only partly on biological grounds which are, themselves, still open to scientific debate. There is wide agreement within the international biological and medical communities that some such limit on embryo research is needed. The 14-day rule probably is as good as any for now. But legislators in Britain and elsewhere should understand that it may later be found either too restrictive or excessively liberal. It will take research on embryos themselves to better define the emergence of humanness.

Also, those who would ban embryo research entirely face a dilemma. Many of these consider test-tube fertilization to be a valid medical aid for infertile couples. Yet, as Robert G. Edwards, who pioneered the technique, points out, more embryo research is needed to ensure the safety and efficacy of the process. In a recent article in Nature, which he co-authored with British attorney M. Puxon, he says, ''To undertake in vitro fertilization without guarding as far as possible against the birth of handicapped children is indefensible. The clinical application of (the technique) demands research on embryos.''

Ultimately, the issue of embryo rights cannot be settled by legal or biological considerations alone. It turns on deeply held spiritual values - on one's concept of man. Edwards and Puxon recognize this when they note that the issues of embryo research ''strike at the root of life itself.'' They explain, ''On the one hand, such research could open undreamt-of vistas of improvement and amelioration for mankind; on the other, it raises fears that, by interfering with life's earliest beginnings, it may threaten the sanctity of human life in general.''

The Warnock committee has supplied a stop-gap answer for an increasingly insistent question. But the question is too important to be left to experts and special committees. It should be pondered in families, by those seeking high-tech aid with reproduction, by religious and social groups, and by people everywhere. It is the ancient question - what is man?