After the death of Henri II in 1559, the development of the Louvre took a new direction, and it wasn't until 1625 that work on the original courtyard resumed.
In the meantime, in 1564, the widow of Henri II, Catherine de Medici, decided to build an entirely new palace to the west of the Louvre, which became known as the Tuileries palace. This prompted a new plan to unite the two buildings into one coherent mega-complex. The Petite Galerie, a small wing protruding from the Louvre courtyard down to the river was linked to the 300-metre long Grande Galerie, which effectively created a corridor between the palaces along the river. This was completed during the reign of Henri IV in the period 1589-1610.
After the king was assassinated, however, building ceased and little happened in the early reign of Louis XIII. In order to resume the grand project, it had been decided to quadruple the size of the courtyard.
The next phase, therefore, returned the focus back on the oldest parts of the Louvre, which still featured half-demolished medieval wings from Lescot's time.
The architect chosen for the job was Jacques Lemercier. In order to enlarge the courtyard, Lemercier first placed a large pavilion at the top of the western, and original, wing by Lescot, and then copied Lescot's design for the continuation of the wing, for the sake of symmetry.
The western wing of the enlarged courtyard was thus complete. At the north-west corner, Lemercier again continued the design of Lescot with respect to the corner pavilion. The principle of sober exterior and richly decorated courtyard was likewise continued. Work was also begun on the northern wing but did not proceed very far during Lemercier's time.
As such, the original design by Lemercier is the pavilion de l'Horloge, so-called because of a clock that was later inserted. The rest of the work by him is effectively a continuation of the scheme by Lescot. The pavilion, however, is a highly significant contribution and one of the most recognizable features of the palace as it stands today.
As did Lescot, Lemercier articulated the pavilion by the use of half-columns as opposed to pilasters, at the two lower floors. This is further emphasized by the triumphal arch-theme of the ground-floor passageway, continued somewhat in the floor above. The outer half-columns are coupled to give the sense of strength and balance. However, the chief means Lemercier used to control the total composition and highlight the prominence of the central pavilion, as opposed to Lescot's smaller pavilions, is height. The highest floor of the new pavilion rises above the old roofline and is capped with a dome-like structure, the first of its kind, along with tall chimneys. The tallest floor is also decorated with caryatids instead of columns, presumably because no order can supersede the composite, which already had been used by Lescot at a floor below. The use of caryatids is also likely a reference to Lescot's Salle des Caryatides inside the Lescot wing. The pediment is triangular, but incorporates within it one segmental and another triangular pediment. Similar motifs can be found in mannerist architecture in Italy, as for example with Michelangelo's Porta Pia in Rome. The chapel, which Lemercier built inside the pavilion is no longer in existence.
In accordance with Lescot's sober exterior, no columns or pilasters are used on the new pavilion by Lemercier either, and only the windows of the first floor have pediments. Instead quoins are used to emphasize the pavilion and passageway. The central pediment below the dome appears to be rather simple. The appearance of this facade, including the pavilion, was altered in the 19th century. The pavilion has also since been renamed Pavilion de Sully.
For more information on the Louvre, see the following links: http://mexichino-jr.blogspot.com/2011/03/lescot-wing.html, http://mexichino-jr.blogspot.com/2011/05/nothing-remains-of-exterior-of-lescots.html.