For once the writing is literally on the wall. Stand at Lenox Avenue and 110th Street and you will see, emblazoned across the blue wooden boards of a construction site, the future of the northern edge of Central Park: 111 Central Park North.

For the moment, nothing arises over this plot of land, but its projected development has the entire real estate establishment giddy with expectation. The preliminary rendering by Schuman, Lichtenstein, Claman & Efron that graces the blue boards of the site suggests a 17-story gray residential slab divided into three bays. A strictly rectilinear series of balconies, somewhat irregularly distributed along the top, will enliven the façade, while, at the base, there appears to be some weak attempt to foster diversity through a variety of framing devices around the balconies.

In truth, 111 CPN is a rather poor example of late modernism, whose idiom seems entirely ill-suited to its context: If ever there were a place where you’d want a postmodern simulacrum of a pre-war building, such as you now find throughout Harlem, this is it.Yet, the important thing about this new development is not its architecture, but its urban implications.

For this structure, if it becomes a reality, will be the tallest thing ever conceived along Central Park North, not counting the two Schomburg Towers at Fifth Avenue, designed by Gruzen & Partners in 1975 and quite good as such things go. In other words, 111 CPN is the first attempt to exploit this huge expanse of park views in the name of development. Unless the city or some community board steps in with other ideas, this doorman building will be the first step in a process that, within a matter of decades, will transform the northern edge of the park, heretofore hardly even on the map, into an area every bit as desirable as Central Park South, Central Park West and Fifth Avenue.

Many of us have had a dream in which we discover in our home a room we never knew existed. Standing on Central Park North, one is put in mind of such a dream. Even people intimately, proudly familiar with the skylines of Fifth Avenue and Central Park West and South, may never have ventured this far up or even entertained the most fleeting curiosity about Central Park’s northern limit.

But if you go there now — and you should go quickly — you will glimpse the way the three other sides of the park looked 100 years ago. Consider that, when Olmsted and Vaux first conceived Central Park in the 1850s, they could hardly imagine that the day would come, scarcely two generations in the future, when buildings would rise above six stories. Aside from cathedrals, which were not likely to grace the park, no one had ever seen a building much taller than a town house. Thus, although the designers correctly foresaw that the park would spur development on all sides, they envisaged buildings no taller than the trees themselves, buildings that would not disturb the bucolic illusion. Never could Olmsted and Vaux have foreseen the twin towers of the Majestic, the San Remo, and the Eldorado, let alone the skyscrapers along 59th Street. On Central Park North, however, this vision survives.

But how much longer will it survive? As you walk along this street, you sense the explosive change that is about to come. It was heralded nine years ago with the dedication of Duke Ellington Circle. Though rarely populated, this circle gracefully inflects the northeastern corner of the park almost as dramatically as the Grand Army Plaza and the Maine Monument transform the southern end. Meanwhile, across the street, is one of the most auspicious new projects in New York in ages: the opening, two years hence, of the Museum of African Art at Fifth Avenue and 110th Street, after almost a generation of nomadic existence. The perfect rhyme of this project will consist in its being the point of contact between the southern edge of Harlem and the northern tip of Museum Mile.

Many other changes are in store. It is a fair guess that the two parking lots on Central Park North, one across the street from Duke Ellington Circle, the other between Powell and Douglass (or Seventh and Eighth avenues) will soon give way to development. How much longer, for that matter, will the two story La Hermosa Christian church survive, or El Templo Cristiano “Bethel,” right next door, or, for that matter, the Altagracia bodega next to that? For the moment, the tallest building on the street is an elegant beaux arts structure that houses the Lincoln Correctional Facility. With high security windows and a cage-like contraption at the top, it will soon see its real estate value increase around 50-fold.

As for much of the rest of Central Park North, it is remarkably well preserved, with an assortment of old structures whose styles range from vaguely classical to Richardsonian Romanesque. The great question is whether these buildings, mostly six stories tall, should be preserved as they are, or whether they should yield to taller structures that will present to the world a northern skyline as ambitious as the three other sides of Central Park. If they are to be preserved — and a strong case can be made for doing so — there will be an irresistible urge to add penthouses. We can only hope these will be applied more gracefully than the one at 125 Central Park North, immediately to the west of the building planned at 111 CPN.

At the western end of Central Park North, where it intersects with Central Park West, a major effort is under way to create a park entrance similar to Duke Ellington Circle on Fifth. It is scheduled for completion — after interminable delays — in August 2007. But we must wonder whether the western end of Central Park North can ever be fully redeemed, whether it will not permanently undermine the best-laid plans of the developers. Surely the gas station enthroned upon its northwest corner can, must, and will disappear. But what about those two hideous residential slabs that make up the Towers on the Park project, designed by Bond Ryder & James in 1987, and that represent the worst excesses of Brutalist poured concrete? Though it is nice to know that, in all likelihood, these slabs would be proof against a hydrogen bomb, they will represent a blight upon Central Park North in perpetuity, as long as they are suffered to remain standing.