  Recent Solar Dynamic Observatory observations reveal that coronal mass
ejections (CMEs) consist of a multi-temperature structure: a hot flux rope and
a cool leading front (LF). The flux rope first appears as a twisted hot channel
in the Atmospheric Imaging Assembly 94 A and 131 A passbands. The twisted hot
channel initially lies along the polarity inversion line and then rises and
develops into the semi-circular flux rope-like structure during the impulsive
acceleration phase of CMEs. In the meantime, the rising hot channel compresses
the surrounding magnetic field and plasma, which successively stack into the
CME LF. In this paper, we study in detail two well-observed CMEs occurred on
2011 March 7 and 2011 March 8, respectively. Each of them is associated with an
M-class flare. Through a kinematic analysis we find that: (1) the hot channel
rises earlier than the first appearance of the CME LF and the onset of the
associated flare; (2) the speed of the hot channel is always faster than that
of the LF, at least in the field of view of AIA. Thus, the hot channel acts as
a continuous driver of the CME formation and eruption in the early acceleration
phase. Subsequently, the two CMEs in white-light images can be well reproduced
by the graduated cylindrical shell flux rope model. These results suggest that
the pre-existing flux rope plays a key role in CME initiation and formation.
