переходная стадия — transition stage
на стадии — at / in the stage (of)
на первой стадии — early in stage, in the first / initial stage, in an early stage
на начальной стадии — in an initial stage (of)
на начальной стадии развития — in a primitive state of development
достигать стадии — to reach a point
в последней стадии — far gone
на более поздней стадии — in / at the later stage
на поздней стадии болезни — late in the disease