THE PURPOSE of economic development in the poorest regions of the world is not to be found in the rows of statistics on production and trade. It lies in the opportunity to improve the chances that newborn infants will survive, that they will go to school and learn to read, and they will live longer than their parents, that they will spend those years in communities more hopeful than the slums now growing uncontrollably around the tropical cities. The World Bank's annual meeting, now in progress here, is always an occasion for taking stock of conditions among the rich and the poor. The bank itself, as the major conduit of development capital from north to south, has also become the leading source of reliable comparative information on subjects reaching far beyond finance.

By the end of this century, if things continue on their present course, there will be no 600 million people around the world living in absolute poverty, at the edge of survival. That forecast is offered by the president of the bank, Robert S. McNamara, as an indicator of need. Taken by itself, the number raises a certain danger that Americans will respond by merely throwing up their hands at the impossibility of doing anything at all about need on such a scale. But Mr. McNamara is making the opposite point: Things are demonstrably changing and improving in most of the poor countries. By the bank's count, about 800 million people - just under two-fifths of the total populations - are now living in absolute poverty in those countries. If the number drops to 600 million over the next generation, it will mean that the proportion has fallen to less than one-fifth. If so much can be accomplished with the present rather modest amounts of aid and government-backed lending, does that not become an urgent argument for doing more?

There are some three dozen countries - India and Indonesia are the largest among them - in which economic output per capita is less than $250 a year. Out of every 1,000 babies born there, 122 die before their first birthday. That's eight times the rate in the United States. But it's down from the 1960 rate of 144. Life expectancy there is about 44 years (compared with 73 currently in this country), but that's up from 36 years in 1960. Also since 1960, literacy in those countries has almost doubled; one out of every four people now can read, and among young children half are going to school. Only one out of 12 goes on to high school, but 15 years ago it was one out of 50. One family out of every four now has access to a safe water supply.

But the savage arithmetic of population growth continues as usual. Although death rates are down significantly in those parts of the world, birth rates are not. There has been substantial improvement in several big countries, but there's no general pattern. Nearly half of this population is under the age of 15. The proportion living in the cities is still low - about one out of every eight people - but it's rising fast. Food production per capita in those poorest countries is a little lower now than it was a decade ago.

Over the turbulent years from 1970 to 1975 - the years of oil crisis and crop failure - it was generally the middle range of developing countries whose economies expanded most rapidly. The rich industrial economics proved more vulnerable and did less well. But it was the poorest countries whose economic growth rates were lowest of all in those years.

Those countries need two things, above all else, from the industrial world. They need access to markets, and that access is reduced every time a country like the United States puts quotas and restrictions on its imports of their products. They also need capital. That is why the World Bank is now asking its sponsors and donors, the governments of the rich countries, to increase its lending capacity. In relation to the size of its own growing economy, the United States is now contributing just half as much as it did in 1960.