The Third World and its relationship with the United States is the subject of a series of reports that will appear in the months ahead. In this initial series, a veteran reporter, who had never traveled outside the United States, explores life in a number of developing countries.* Three children - all under four years of age - sit beside a small, open trench that passes for a sewer.

A large plate rests between them in the dust. A dozen chunks of sugar cane are on it, covered with a strange red-and-green fungus. Mohammed Taj, a doctor in Frontier Colony, an illegal settlement on the outskirts of Karachi, picks a ball of stale rice from the plate and breaks it open.

Two insects crawl out.

"This is what our children eat every day," he declares. "They collect it from other people's garbage. Look at it. Feel it."

It was my last day in Pakistan, the end of my sixth week in countries Americans refer amorphously as the Third World. I had spent the morning on a round of last minute errands. Everywhere I turned, I ran into beggars - more than I remembered from any other day.

They were grotesque, pitiful creatures: a man with no arms: an elderly woman with one side of her face eaten away by disease: a teen-ager with no legs who propelled himself about on a square board with two casters.

And the children. Always the children, holding out their tiny hands.

Two of them followed me for almost a block at one point, first holding out their hands, then bowing in front of me. Finally, I gave one of them the equivalent of 50 cents telling her to share it with her companion.

She didn't. The other girl dropped to her knees, kissing my feet.

"No momma. No poppa. No food," she chanted.

A visitor to the Third World returns with dozens of such encounters - snapshots, if you will - etched on the mind. Exotic snapshots, contradictory ones. Snapshots of poverty and wealth, of beauty and ugliness.

Individually, they offer no great truths. Collectively, they tell what it is like for one American to come face to face with a bewildering part of the world, bound together by its colonial past, poverty and quest for survival.

We've all read the statistics: 400 to 600 million persons on earth, according to World Bank estimates, have diets sufficiently inadequate to retard brain development: 70 million persons annually face starvation; more than 1.1 billion humans - one quarter of mankind - live in countries where the annual per capital income in 1977 was less than the $265 an American auto worker earns in one week.

We know there are many poor people in the world. We know there are children who go to bed hungry. But what is hard to comprehend is the enormity of their poverty, and their potential for social and political turbulence.

Put yourself for a moment beside a Masai tribesman in the backseat of an auto bumping across the arid plains of northern Tanzania. He is a tall, lean herdsman wearing only a tomato-colored cloth draped over his body. A huge ring hangs form one ear. His seven-foot spear pokes out the window.

He speaks no English, no Swahili. You can't communicate. Even if you could, what would you talk about? Here is a man closer to the Stone Age than the computer age. How do thousands of people like him survive in the late 20the century? What will become of them in the years ahead?

Poverty, disease, unemployment, overpopulation, illiteracy, lack of opportunity. These are the common threads that link the poor of the Third World.

In some ways they are like the poor from the hills of Appalachia, the small towns of the Americans South or the North's big city ghettos. The rows of houses tht plantation owners built for their workers in the coastal plains of Guatemala look strikingly like the ones coalcompanies built in Eastern Kentucky and West Virginia.

By and large, the poor of both the developed and the underdeveloped world are very practical people. They live hand-to-mouth existences, getting by on their native common sense and an uncanny knack for survival.

What is different, however, is that poverty in the Third World is so absolute, so total. The poor are not a forgotten minority; they are the vast majority. Poverty is an accepted fact of life, something the poor for the most part feel helpless to change.

Their ranks are growing at a phenomenal rate, straining the world's ability to read, house and clothe them to a near-breaking point.

By way of comparison, the population of the United States and Canada is expected to increase by 140 million during the last half of the 20th century. The population of Africa, however is expected to grow by 535 million. Latin America by 440 million and Asia by 2.1 billion.

Their numbers combined with the nascent political awakening of this global majority are daily bringing pressure on the world's "haves" to agree to one of the most intensive transformations of the international political and economic order in history. Their demands for food, water, living space and human dignity are expected to dominate the global stage in years to come.

Today, the majority of them are children. Far more than half the poor of the Third World are under 19. In Pakistan, 46 percent of the population has yet to reach its 15th birthday.

In America, we pamper our children. We sacrifice so they can go to the best schools. We plan for their futures. We try to protect them from reality.

In Karachi's Frontier Colony, children today live the same life that Charles Dickens wrote about in London in the 1840s.

The Frontier Colony is one of four "unauthorized zones" which adjoin one of Karachi's largest industrial areas. "Unauthorized" means the city government doesn't recognize that the area and its 400,000 people exist. Therefore, it provides no city services. No schools. No sewers. No garbage pickup. No hospitals. Nothing.

There are places like it in almost every major city in the Third World, varying only in degree of deprivation. They share the same rhythm of life, the same diseases and the same heavy stench of poverty.

Large families live there in two-room shanties, often constructed of mud, rocks, corn stalks or cast-off scraps of lumber. They cook their meals over open fires.

The children live on the street. They play amid the garbage, dangling their feet in the raw sewage that flows past their doorsteps. Flies are everywhere.

A boy of seven places a bagful of dirty bones, some with rotten flesh clinging to them, in a streetside scale.

A shopkeeper hands the boy a coin, then throws the bones into the room behind him.

The room seethes with flies. It is filled waist high with a gruesome collection of bones from dead cows, dogs, sheep, water buffulo and lord knows what else.

The shopkeeper says he sells the bones to the chemical company. Most are collected by children going through garbage. On a good day, an ambitious kid can earn 10 cents.

Two streets away, four tiny girls gather cow dung from a cattle pen, carrying it in the lap of their skirts. They say they'll mix the dung with straw, form it into patties, and dry it. Then, they'll sell it for cooking and heating fuel.

Each child seems to suffer from some life-sapping disease. This one, Dr. Taj tells me, has malaria. That one TB. Another one tapeworm and a scalp disease.

"Becuase they don't have enough money for food or medical care, most of the people are sick," says Taj. The mortality rate, he adds, "has tripled during the last three years."

There are two separate and distinct societies in Pakistan and much of the Third World: one desperately poor; the other affluent, educated, well-fed and often Westernized.

The gap between them seems far greater than the gap between the slums of Anacostia and the mansions of Foxhall Road.

One reason is that there is still so little middle ground in the developing countries. A middle class is just beginning to emerge in many nations, and where it does exist, it tends to ally itself with the affluent.

The affluent isolate themselves from the poor. They build walls around their homes and place guards at their gates. They don't talk about poor people - they talk about peasants. And the subject seldom comes up, except when they complain about their servants.

How can they stand to live as tiny islands in such a vast sea of poverty? The answer most often comes with a shrug: "After a while, you don't notice it."

My reaction was almost as disturbing. I noticed too much.

I found myself growing calloused. I resented the beggers and the diseased children. I resented their poverty, and how it played on my Mid-western conscience. I resented how they intruded on my life.

After a visit to a slum or backwater village, I found myself retreating to my hotel, often for hours at a time.I made excuses, telling myself I was tired, or that I needed to write a letter, or get another meal under my belt.

The truth of the matter was that I'd seen enough. The snapshots, and the sense of hopelessness they portrayed, kept turing over in my mind.

I had left Washington knowing little about the Third World. I had never traveled outside the United States, but I had spent months working among some of the very poorest people in America. I had shared their meals and their homes. I'd written about children starving to death in the slums of Louisville.

But I found life in America hadn't prepared me for my encounter with the Third World.

Scores of people were waiting in the dusty market square of Dodoma in central Tanzania when I arrived before dawn to take the bus to Dares Salaam. I bought ticket number 59 on a bus that supposedly sat 60.

By the time the bus arrived, 45 minutes behind schedule, it was obvious that far more than 60 people wanted to get on it. A huge crowd - women with babies strapped to their backs, men carrying chicken crates - rushed for the bus door.

I dove into the middle of it, fearing that I wouldn't get on the bus and would be stranded in Dodoma for days. Everyone was belly-to-belly, pushing and shoving. I felt a strong tug at the strap on my shoulder bag which contained $2,500 in airline tickets, two cameras and my passport. Someone was trying to steal it! I grabbed hold of it, spun around and shoved my way onto the bus.

For a moment, I felt relieved. Then I felt the empty pocket in my jacket and the empty pocket in my blue jeans. My wallet with all my money and credit cards was missing! So were my travelers checks.

I leaned out the bus window, yelling that I'd been robbed, and that I'd give a reward to anyone who returned my credit and identification cards. They could keep the money. No one seemed to pay any attention.

(I subsequently found that only three people on the bus spoke any English. Two, concerned over my plight, invited me to stay in their homes.)

A moment later, I heard shouts coming from h the front of the bus. A tough-looking man was accusing a teenager of being the pickpocket. He threw the teenager against the seat, jumped on him and began savagely beating him about the head. A large cut opened under the teenager's eye. Blood poured from his nose. There was shouting all around. I could feel the tension building in the bus. I frisked the suspected pickpocket, and let him go.

Now I faced a dilemma. The bus motor had started. Should I get off and try to find my wallet? If I did so, I had no my to get out of Dodoma. I only had 28 shillings, about enough for one meal.

The previous day, it had taken me 50 minutes to cash a $50 travelers check at the state-owned bank because it had to be approved by seven different people, who filled out nine separate forms. There was no assurance that the police would be any more efficient.

If I left, I would give up all hope of getting my credit cards back. I decided to stay on the bus.

The trip took a little over 12 hours. The bus broke down four times. Sometimes, it would go half an hour without passing another vehicle. Every 90 minutes or so, it stopped at a village. They were unbelievably primitive. Almost every home was crudely constructed of mud and sticks. Farmers worked their fields with hoes: some carried spears as they herded their cattle. Women carried jugs of water on their heads.

I felt terribly alone in a strange lanr on their heads.

I felt terribly alone in a strange land.