layout of the buildings was also novel, since they did not continue the tripartite plan inherited from the Ubayd period: buildings on the Eanna at this time had labyrinthine plans with elongated halls of pillars within a rectangular building. The architects and artisans who worked on these sites this had the opportunity to display a high level of creativity. Means of transport A debated question in the realm of transport is whether it was in the Uruk period that the wheel was invented. Towards the end of the Uruk period, cylinder seals depict sleds, which had hitherto been the most commonly depicted form of land transport, less and less. They begin to show the first vehicles that appear to be on wheels, but it is not certain that they actually depict wheels themselves. In any case, the wheel spread extremely rapidly and enabled the creation of vehicles that enabled much easier transport of much larger loads. There were certainly chariots in southern Mesopotamia at the beginning of the 3rd millennium BC. Their wheels were solid blocks; spokes were not invented until c. 2000 BC. The domestication of the donkey was also an advance of considerable importance, because they were more useful than the wheel as a means of transport in mountainous regions and for long-distance travel, before the spoked wheel was invented. The donkey enabled the system of caravans that would dominate trade in the Near East for the following millennia, but this system is not actually attested in the Uruk period. For transport at the local and regional level in Lower Mesopotamia, boats made from reeds and wood were crucial, on account of the importance of the rivers for connecting places and because they were capable of carrying much larger loads than land transport. City-states The 4th millennium BC saw a new stage in the political development of Near Eastern society after the Neolithic: political power grew stronger, more organised, more centralised, and more visible in the use of space and in art, culminating in the development of a true state by the end of the period. This development came with other major changes: the appearance of the first cities and of administrative systems capable of organising diverse activities. The causes and means by which these developments occurred and their relationship to one another are the subject of extensive debate. The first states and their institutions The Uruk period