If by "suburb" is meant an urban margin that grows more rapidly than it already developed interior, the process of suburbanization began 41______ during the emergence of the industrial city in second quarter of 42______ the nineteenth century. Before that period the city was a small highly compact cluster which people moved about on foot and goods were 43______ conveyed by horse and cart. But the early factories built in the 1840’s were locating along waterways and near railheads at the edges of cities, 44______ and housing was needed for the thousands of people drawn by the prospect of employment. In time, the factories were surrounded by proliferating mill towns of apartments and row houses that abutted the older, main cities. As a defense against this encroachment and to reduce their tax bases, 45______ the cities appropriated their industrial neighbor. In 1854, for example, 46______ the city of Philadelphia annexed most of Philadelphia County. Similar municipal maneuvers took place in Chicago and in New York. Indeed, most great cities of the United States achieve such status 47______ only by incorporating the communities along their borders. With the accelerate of industrial growth came acute urban 48______ crowding and accompanying social stress -- conditions that began to approach disastrous proportions when, in 1888, the first commercially successful electric traction line was developed. Within a few years the horse-drawn trolleys retired and electric streetcar networks crisscrossed and connected 49______ every major urban area, fostering a wave of suburbanization that transformed the compact industrial city into a dispersed metropolis. This first phase of mass-scale suburbanization was reinforced by the simultaneous emergence of the urban Middle Class, whose desires for homeownership in neighborhoods far from the aging outer city were satisfied by the developers of single-family 50______ housing tracts.