Where was the Susan van Dyne analysis taken from?

In the Sylvia Plath Poems summary and analysis of 'Ariel' article, where was this interpretation initially taken from?

'Susan van Dyne notes how the poet's self-transformation is manifest in her use of complete sentences, which begins midway through the poem. She becomes both male and female, horse and rider, poet and creative force, arrow and target. She is not merely a captive of the creative drive, but its agent.'

Thanks.