  In the framework of the model assuming the formation of NUC on the pairs of
Cu ions in CuO$_{2}$ plane the mechanism of hole carrier generation is
considered and the interpretation of pseudogap and 60 K-phases in
$YBa_{2}Cu_{3}O_{6+\delta}$. is offered. The calculated dependences of hole
concentration in $YBa_{2}Cu_{3}O_{6+\delta}$ on doping $\delta$ and temperature
are found to be in a perfect quantitative agreement with experimental data. As
follows from the model the pseudogap has superconducting nature and arises at
temperature $T^{*}>T_{c\infty}>T_{c}$ in small clusters uniting a number of
NUC's due to large fluctuations of NUC occupation. Here $T_{c\infty}$ and
$T_{c}$ are the superconducting transition temperatures of infinite and finite
clusters of NUC's, correspondingly. The calculated $T^{*}(\delta)$ and
$T_{n}(\delta)$ dependences are in accordance with experiment. The area between
$T^{*}(\delta)$ and $T_{n}(\delta)$ corresponds to the area of fluctuations
where small clusters fluctuate between superconducting and normal states owing
to fluctuations of NUC occupation. The results may serve as important arguments
in favor of the proposed model of HTSC.
